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What planetary conditions would have to exist to make computers and other complex electronics useless or at least difficult to maintain?
Would ionised atmospheric conditions, constant solar flares, and unusual elements affect electronics and would these things damage humans or make human life unsustainable?
First a look at external (solar) events:
A lot of things that destroy computers also destroy humans too. :D You are right in that a heavily ionised atmosphere which would be caused by constant solar flares would interfere with electronics. Constant solar flares would also strip away your planet's atmosphere unless it (like Earth) has a magnetic field protecting it. Presumably, your terraformed planet already has a magnetic field (or it wouldn't be terraformed) so solar flares probably aren't much of a problem.
A Coronal mass ejection (CME) would be far more dangerous to technology. This page has relevant information. Basically, a CME is a huge fluctuation in a star's magnetic field that can cause your planet's magnetic field to go crazy and also fluctuate. This causes any conductor (power lines, etc) on your planet to become an inductor, ie: induce electricity and cause massive overloads in electrical systems.
Planet side events:
As Erik noted in the comments, a lack of necessary resources, such as semiconductors or metals would be the easiest way of preventing the development of computers. A strong fluctuating magnetic field could also affect electronics. Perhaps your planet has microbes that have a taste for coppers and silicons. Cosmic rays can already (rarely) flip bits in a computer's memory and corrupt data. Radiation (especially 'hard' radiation like X-Rays, Gamma radiation, etc) can also seriously damage computers, perhaps your planet or star is surrounded by a radiation field that passes through humans but can affect electronics. A commenter has mentioned ECC memory for protecting computer memory against errors and I would also like to mention Radiation hardened electronics. Both these technologies can help protect electronics from malfunction but are not (yet) used by the average Joe due to cost and several other trade-offs, such as complexity and reduced compute density (the computer isn't as fast). They are also used on spacecraft. Your planetary conditions must somehow mitigate these technologies, either with extreme radiation, magnetic interference and/or bacteria. It is probably impossible to completely get rid of computers but with enough counterpoints, they might be a rare sight.
Ideology could also affect the development and use of computers in your world. Perhaps your colonists dislike complicated or hard to understand machines. I have less information on this but you might take a look at the Luddite riots and Amish culture.
Temperatures, computers does not run well under extreme temperatures, or extreme weather conditions.
Most of those conditions on earth is preventable. Though even on earth buildings are flooded, computers near equator is overheating due to lack of cooling, most people have either tried them selves or know someone who had a computer / tv / modem or switch burn from lightning, and special equipment is used for polar exploration and military use.
Now make those conditions even more extreme - and you got an environment where the computer has a hard time to function.
If it affects humans ability to live? Well people are surviving on the south-pole, still people living in the rainy parts of India, and dessert people is also a known fact. So it might not be easy to live under those circumstances, but i wouldn't say it was impossible.
Many physics related and social causes have already been mentioned, as well as copper-eating microbes.
Into this mix I'd like to add pests in general as factor. Especially small ubiquitous bugs or moulds attracted by silicon & electric current will make cheap and easy electronics impossible.
A variation would be a common disease making humans allergic to same.
Conditions that would prevent coherent electrical circuits would definitely work. Electromagnetic radiation (maybe even periodic EMP bursts caused by a rotating planet core) would make it impossible to keep things going. A lack of materials is also an option. Earth is unusual among the planets in terms of how much metal it has. Without conductive metals, electronic technology would find it difficult to evolve.
Another option is the one followed by the Coldfire Trilogy where an aspect of the environment means that doubt has an influence on how things work. Enough people don't understand computers that they'd be rendered useless.
If you want something that doesn't otherwise significantly affect people's lives, your best bet is lack of electronics manufacturing capability (and of the ability to import it).
Most of our current advanced electronics have fairly short design lifetimes, because they become obsolete so quickly. This holds true for a handheld device as well as internet infrastructure; all of the myriad chain links that make the Internet possible need constant maintenance and replacement, which would be made difficult-to-impossible if you couldn't get replacement parts within, say, a month.
If you want to exacerbate the issue further, add omnipresent dust. If it gets inside a device, it will cause components to overheat/melt and connections to fail, so you'll have fairly short lifetimes even on simpler electronics than your average iPhone.
It will still be possible to have environment-proofed devices, but those won't last forever and once broken will be impossible to fix (because if you open them up, the dust gets everywhere). And they will not be connected to any kind of "internet", because that would be impossible to keep operable.
This is a fairly mundane solution that rules out most electronics, but still allows some capability for those who are rich enough to get the proofed versions and keep them in operation.
I wonder why it was not mentioned before, but your best bet is probably social reasons. In the novel called Dune, written by Frank Herbert, it is forbidden to construct any kinds of "computers, thinking machines, and conscious robots". Instead, they use a special substance called "the Spice" to enhance human thinking capabilities.
The reason is simple distrust -- humans were afraid of the thinking machines and destroyed them all.
ionised atmospheric conditions
The dead sea has the most saline atmosphere in the world. The second place goes to Brazil's fifth largest city, Fortaleza. I come from there.
The atmosphere is saline because humidity is not water in a gaseous state, but rather water droplets in suspension. These droplets can hold salt within.
Now usually for environments such as coastal ones, manufacturers varnish circuit boards. This adds to the cost of manufacturing. But doing so is not a complete solution. In my home city stuff just lasts less than elsewhere. Electronics, appliances and even car paint will eventually corrode really fast if they collect dust, or if they are exposed to even minimal amounts of wind or humidity.
By making your world briny everywhere, you ensure that devices have to be replaced every few yearsdue to nature rather than due to consumerism.
