There are some issues with creating signature in User Management when a user does not have full privileges. In addition, at X University, with full privileges, user had issue copying and pasting into the signature window. Might have been some funkiness with the "double click to enter signature". When I double clicked (with full privileges), I got that error highlighted in red that says something about broken plugin and then, just closed it out and things worked.
Hi Jodi,
Adding as a related ticket to Bug 14817. Although the behavior isn't exactly the same, it probably is related and the developers will review along with the permissions issue for Basic Users.
Kathy