I performed an upgrade of Connections from 5.0 to 5.5 for a customer, and in the new environment Files were not working correctly using the web UI. When a user clicked on Files the page would get stuck showing “Loading…”.
In a chat with Michael Urspringer from IBM I found he had stumbled up on the same issue and that he solved it uninstalling iFix LO87906. So I did the same and the problem was gone; there is no official from IBM Support at this time, but the fact that two installations had the same problem and solved it by removing the fix leads toward the identification of LO87906 as the culprit.
I have other installations, both new and upgraded from 5.0, where this does not happen so it seems that the fix breaks Files only in some specific circumstances I have not been able to identify, in any case if you see Files not working correctly it would be worth removing that fix and see if this solves the problem.