Time Machine stopped working

https://preview.redd.it/5l8cc94ics6e1.png?width=540&format=png&auto=webp&s=69d4ee453d8c96cc01453af5ad4f28798def6c6f I've had a Macbook for a year now (1TB), and set up Time Machine soon after getting it. After seeing that it was using over 3 TB on the NAS, I decided to have a look and set a quota (tmutil setquota) to just 2 TB and... now it hasn't worked for 3 weeks. "*Unknown error*" is not helpful. I had hoped it would do its thing and delete older snapshots, but apparently it's not how it works. Note that the folder on the NAS is still using 3.35 TB, and there are 6 TB free now. I don't understand the 2.05 TB available, and I can connect to the NAS just fine from Finder.