When you attempt to remove a tag from a category, the following error is displayed:
An error occurred on the server. Please try again and if this problem persists, please log a bug.
java.lang.IllegalArgumentException: id to load is required for loading
This is caused by changes in BZ#1116705, so going to mark it as a duplicate of that.
The cause of this is that it previously used to use the category id to remove the tag, but I changed it to be part "relationshipId" which isn't set by the UI. As such I'll update the rest server to check if relationshipId exists and if so use that, otherwise use the category id.
*** This bug has been marked as a duplicate of bug 1116705 ***