After my Synology NAS updated itself to version 6 one of my Time Machine backups stopped working. It might have been coincidental. Time Machine claimed I only had 350GB free and it needed 1TB, but Synology claimed I had enough free space.
The fix was to increase the quota size for the user who owned the Time Machine disk image belonging to my MacBook Air.
Except I couldn’t do it. I could edit the user easily, but the quota information couldn’t be edited. Clicking on the row did nothing.
Google helped. It’s a UI issue. There’s nothing in Synology’s UI to tell you to click specifically on the quota number. If you do that you can edit it.
It’s probably a good idea to turn Time Machine backup off while you’re doing this. In any case it’s fixed my problem.