I have experienced this issue once in the past, The strangest thing resolved the issue for that user. They had to log out, log into another project and attempt the same actions that worked and then logging back into the affected project resolved the issue. I can only think that something was initially corrupted or changed in the file location for that project and the action of switching the projects cleaned up and corrected the issue. I would attempt this as it does two things... it may correct the issue as in the other instance and if not it tells us if the issue is project related or user account/possible client system related. If it fixes it ...great. If not it provides background for further trouble shooting. If the other project has the issue then it could be either system or user account related. If it doesn't resolve it but the other project works then I would recommend attempting to remove and re-add the user to the project and retest.
I would next if the issue isn't resolved have the user attempt the actions on a known good other system. If they work then the issue is related to the client's local system. If they don't then remove the user from within site admin and then add the account back as the user's account may have gotten corrupted (also be sure and add the user back to all projects), then test if the issue is resolved.
If you still have issues I would recommend opening a case with your support team and you have some good information on what you done to test the situation and possible resolutions attempted.
Hope this helps,