I am getting this error pretty regularly. I'll see a message in my inbox, and when I tap through to view it in context, it's missing. Can't find a cause or a workaround.
Ok, I have more info on this bug:
When I have comments sorted by "hot" or "top", I get the error I mentioned above. Tapping the comment in my profile or inbox takes me to the comments section of the post, but often gives the "could not find comment key..." error.
When I have them sorted by "new" or "old", I don't get the error. Apparently, the comments I'm clicking on in my inbox and profile don't fit the criteria for the "top" or "hot" algorithms.
Changing the default sort is a good enough workaround for me, but this is a pretty annoying bug.