Time Machine stops after a few gigabytes, without any visible error message
Solution 1:
Problem seems to be solved, I am not sure how.
- I deleted the erroneus file, but I don't see why it would cause problem in the first place
- By a friend's suggestion I've moved physically closer to the Time Capsule, maybe it is getting older and the wifi connection is weaker
- I've deleted a volume from boot APFS container (but it wasn't mounted anyway)
I am still somewhat puzzled, but maybe someone coming by with same symptoms can make use of any of these.