I had a problem, where I couldn't create new Epics, because the "Epic name is required" field would pop up, even though the field has been filled.
I realized that the cause of this problem is the fact that I have ~10 custom fields called "Epic name". I followed some instructions on how to delete them and they worked, but some fields keep reappearing.
I looked into it and found that under Administration > Issues > Field configurations > Default Field Configuration there appear to be multiple duplicates of this field.
The problem is, of course, that Epic name is a required field, so I have to fill as many Epic name fields as there are in the configuration, which is annoying.