As mentioned in the comments you will need to engineer your apocalypse in some specific ways to make this probable.
The faster the apocalypse happens the better. As @Kepotx mentioned printed media can be produced astonishingly quickly in the modern day and age. The longer an apocalypse takes to happen, the more time there is for media about it to accumulate. It's highly unlikely that no media will be produced about something dangerous enough to wipe out a significant portion of society, but the faster and more unpredictable it is the better from your perspective.
One way to mitigate this is to choose the right locations to set your story in. You have two options here.
The first is to pick somewhere poorly developed, or somewhere very isolated. Somewhere you are highly unlikely to find large numbers of newspapers littered about (or alternatively somewhere that newspapers are very unlikely to survive for any time post-apocalypse). Somewhere like the Congo is an ideal option. Large numbers of isolated people, poor transport infrastructure, wet climate that means any existing newspapers aren't likely to survive. This also has the benefit of the population having a lower literacy than the West, so any printed media that does survive is less likely to be interpreted correctly.
The second option is to engineer the world of 2100 such that print media has spectacularly fallen out of fashion for news. It's already a process that's sort of underway with the rise of digital media. All you need to do is shift it along a bit quicker, which isn't all that unbelievable in an 81-year timespan.
You've already touched on this, but this is also important. Even without print media, people are very good at maintaining oral histories with a reasonable degree of accuracy. For instance, Inuit oral history is phenomenally accurate, and has been used to untangle previously unknown historic events.
The apocalypse is a massive thing. People are going to remember. For reference, it's likely that the high instance of flood myths in a large variety of religions are echoes of a widespread prehistoric flood event; either the flooding of the Mediterranean or the Sea of Azov are prime candidates. Stuff like that tends to stick around.
Some of this can be solved by location. Pick somewhere detached enough and the people aren't going to know in the first place. Somewhere like the Congo, or perhaps a closed state like North Korea if they continue to be paranoid into the 22nd century.
Another way to solve this could be to force your population through a historic bottleneck sometime post-apocalypse. The fewer people there are, and the younger they die, the less likely it is that an oral history will survive intact. This also has the added bonus of making it less likely that your people will be literate. It takes a lot of investment to teach someone to read, which may not be feasible in a pressured environment. This is also fairly likely as the population adjusts to a new and presumably dangerous environment.
If you touch on all of these effects then as far as I'm concerned it's eminently believable that your characters won't have a clear understanding of what caused the apocalypse, with two caveats:
Firstly, it's important to remember that the understanding of what happened will vary across the different populations of the globe. Some will have access to more stable information, some will maintain their oral histories better, some will start from a greater level of knowledge to begin with. However, if the transport and communication infrastructure is ruined, there is no good way to share the remaining information with distant populations.
Secondly, it's important to understand that even among the populations that do not remember precisely what caused it, they will still have some inkling that their world has changed massively from what came before, and they will try to rationalise and explain it.
What those explanations will be will depend on a myriad of different factors (prior knowledge of the event, how obvious the cause was to the layman, memories of other apocalypse fiction, cultural history, religious history, some random guy in the dim and distant past making up a story that stuck in people's minds etc.).
Hope that helps!