Trying to edit a server-based termbase definition will throw one of the following error messages at the end of the process:
Role 'Public' does exist in termbase.
Parameter name: roleName
Role 'Expert User' does exist in termbase.
Parameter name: roleName
This can happen either if you are trying to add new languages to the termbase, new fields (of any type), or new picklist values for an existing picklist type field.