Just putting this here in case anyone else comes across the error.

Steps:
1. Add a new tag, including a square bracket in the name
2. Click Save
3. Left click on it to activate the inline renaming (don't change the name)
4. Click off to deactivate the inline renaming

This results in the rather misleading error of:

Violation of UNIQUE KEY constraint 'IX_Term_Unique'. Cannot insert duplicate key in object 'dbo.Term'. The duplicate key value is (en, test2 [).

Renaming via the Edit button doesn't result in the error. I picked this up when dragging tags, which also seems to trigger the same error. Removing the square brackets using the Edit button seems to solve the problem.