Not so distant early warning ...
The next beta release will change how user-defined tag categories (not user-defined columns) are stored, in order to make them available to the calibre server. The effect: tag categories you have defined will go away, and you must create them again.
If this is intolerable for someone, please let me know. One can hand edit the configuration files and preserve the user-defined categories, but the process isn't completely trivial. I would rather work with individuals than try to post general instructions.