Let's start with a bit of a Frame Challenge: you really don't want perfect digital currency
It's important for you to understand that you can always ascribe more magic to digital currency. You can declare it to be secure, less costly to use/process, better synchronized, etc., etc., etc. If you're going to do that, then why ask the question?
Hard currency Will always permit particular forms of fraud, not the least of which is tax evasion, which no government wants. It is, indeed, unreasonable to assume that governments want hard currency. All that instantaneous communication means equally instantaneous control and accounting because governments can more easily mandate the operations of a business than an individual (this becomes really important in a moment).
So if your story needs hard currency you want to avoid perfecting digital currency like you would the proverbial plague. In fact, it might be more efficient for you to ask a second question: what weaknesses in digital currency are justifiable in an interstellar civilization.
And if you think about it, there's no such thing as a completely cashless society anyway
Unless you only think of "currency" in terms of pieces of paper and bits of metal. Value is assigned to everything from real estate (the land my house sits on) to cows (which I intend to BBQ sometime this week) and my time (which I do not have enough of). The direct exchange of these valuable items is usually called "barter," but that's just a word that separates that particular kind of exchange-of-value system from another.
To make matters worse, there will always be concepts having intrinsic value that I would find a challenge to believe would ever become 100% digital (and this coming from a dyed-in-the-wool EE): contracts, stocks, bonds, etc. These are convenient outside-of-government-control representations of value that people stick in fireproof safes and safety deposit boxes because the potential loss of value due to the EMP of a nuclear strike (or government or Google snooping) is... well, it's devastating.
So, from a very real and practical perspective, there has always been and always will be "hard currency." It only depends on whether the two or more people involved in the transaction agree that the "hard currency" is appropriate for their business transaction.
Having demonstrated that physical currency in some form or another will always exist, it's trivial to jump to fiat currency (pieces of paper and bits of metal). Why? Because actually getting rid of the physical aspect of value transfer requires monstrously egregious use of Clarkean magic! Yes, you think it's digital... but you're using plastic cards, your phone or tablet, something completely physical to process that digital information. It's not like you can telepathically verify a financial exchange. Digital information will always require a physical manifestation.1
Which is just another kind of "fiat money," it just has a variable value and the physical manifestation, which must exist, doesn't change hands.
But why is this important? Wouldn't the proverbial debit card be more convenient?
One of the funniest moments of my life was when I had set up a new retail store for my employer and a teenager with a debit card tried to buy 89￠ worth of product. The chain had a \$3 dollar limit for using cards. He grew angry and tried to explain that we'd lose all kinds of money for not allowing transactions of the type he wanted. He was too inexperienced to realize that the cost to us to process that card for such a small amount meant we were guaranteed to lose money on the transaction.
Processing fees will always impose a practical minimum for digital currency exchange. Oh, stores (etc.) may suck it up and take the plastic (or bitcoin, or anything else) at some loss of profit, but they'll never be happy with it. And there will always be processing fees.2 People don't realize that the exorbitant cost of manufacture of a humble penny compared to its intrinsic value is irrelevant! The transactional value3 over the lifetime of the penny is thousands of times greater than its cost of manufacture. And once that penny has been released into the wild — it remains a penny so long as it exists. There is no additional cost to personally hold onto it or to use it in any transaction. Yes, a bank may charge you to hold those pennies for you ... but that's the point: once you convert to digital money, there will always be fees somewhere. Said another way, computers always have maintenance and operational costs, whereas a penny in your pocket doesn't. Therefore...
Physical currency maximizes profit and will always be preferable, even if government would sell its soul to get rid of it to maximize its tax revenues.
Security is relative, meaning that digital may be convenient, but it's anything but secure. Digital currencies like Bitcoin are (theoretically) impossible to track, but they also represent a total amount of GDP value so small it's frankly zero. As soon as cryptocurrencies begin to make a substantial dent, government will once again step in and force tracking. And the moment you do that, security goes out the window.4
Worse is the fact that digital currencies always rely (and will always rely) on some form of pass-coded system. A card+PIN, a phone+password, a cryptocurrency wallet+key. In short, while they're a bit more difficult to steal5 than the dollars in your wallet, when they do get stolen, you lose much more than a few dollars. Remember, we're not looking for Clarkean magic to justify digital solutions, we're looking for practical weaknesses that rationalize continued use of hard currency.
What you're using the money for matters. If you're stashing it away, digital is a horrible solution (physical financial instruments can't be erased with magnets6). If you're just going out for dinner and a movie, it's simply unkind to the business to use a card. On the other hand, large rapid transfers have been "digitized" (from the perspective of a wire transfer) from a time long before "digital" even existed.
Like valence electrons, there are bands of "value" that are simply easier to use via one method than the other. A \$5 burger honestly makes little sense using digital (processing costs!). A \$25,000 car makes even less sense using hard currency. I want my \$250,000 bearer bond neatly typed on a piece of paper in my safety deposit box where nobody can get at it easily! But a billion-dollar business buy-out simply requires the facility of a digital escrow.
Valence electrons... yeah...
But in the end, your population may simply demand it. There's something emotionally grounding about holding "real money" in your hand — and no matter how large the percentage of the population that is completely comfortable with never seeing printed money again (like printed books and vinyl records), there will always be a chunk of the population that simply wants the security blanket of that crisp Benjamin! (Or book, or vinyl record....)
And the government may actually permit it (with careful and prudent regulation, of course), because by that time 99% of all transactions may be digital, and so it's willing to take the "loss" of tax evasion because the comfortable happiness of the interstellar population may actually be a bargain at twice the price.
I'm going to add one more, which I didn't think about before. Anonymity. If you can't figure out reasons why this might be valuable, I'm not going to help you.
1 And only humanity at its best could claim that society is free from the physical hassle of printed/minted money when they still need to pull their phone from their pocket (or at least have it with them... bluetooth...) to buy a Big Mac. What a coup! The government no longer need shoulder the cost of printing the money! The idiot population will buy their phones themselves!
2 The cryptocurrency purist will point out that cryptocurrency operates on donated CPU power. Right you are! Financial institutions around the world will be thrilled that the population is willing to relieve them of the burden of expensive server farms. It's not a processing fee anymore, it's a mandatory minimum CPU/Bandwidth requirement for individual households to ensure that everyone has the equal benefit of a secure digital currency. If you think this has the look-and-feel of a certain national health care policy, you're paying excellent attention!
3 I've met people who don't get this, so let me explain. Yes, it costs far more than a penny to manufacture a penny. But if you add up all the times in a penny's life that it's used to represent one cent (the transactional value), that penny was used to represent thousands of dollars — which is much more than the cost of the penny. If you consider the same issue from the perspective of electrons, the same number of electrons must be pushed around each and every time a "penny" of digital currency is used, substantially lowering the transactional-value/cost-of-use-or-manufacture ratio. Somebody's paying for all those electrons... and it's not the power company.
4 To all the promoters of cryptocurrency: when it comes to software, where there's a will, there's always a way — and few entities on Earth have a bigger will than a government. If you seriously think Bitcoin and its ilk can avoid government oversight, you just hold that happy thought as long as you can. Or hope it never actually becomes popular.
5 Biometrics obviously ups the game, but I saw this wonderful episode of The Blacklist where James Spader invaded an airplane to pull some dude from cryogenic freeze just to cut off his thumbs to get at his safety deposit box, which was biometrically secured. Oh, yeah... it can be done! Hollywood has proved it and Hollywood is always right!
6 There's probably a statistic that claims that fiat money stuffed in a mattress is less safe than numbers stored on a proverbial index card at your local bank. Which is why we don't actually need the FDIC in the U.S. anymore, right?