Okay, but what about the errors below:
Mar 12 15:23:03 Hades kernel: ------------[ cut here ]------------
This seems to be some panic event (major error) or did I understand wrongly?
Anyway, thanks for your suggestions @tbg . I am currently testing your proposed solution.
I wasn't however able to delete all the snapshots. One of them was "not empty" and refused deletion.
I guess that's related to the topic of Docker and BTRFS (described [here])([btrfs] Docker and Subvolumes)
I don't use the docker massively at this moment, so the disk exhaustion isn't extensive, but probably I will need to try the approach described by the author of that post (reinstall the system and setup other filesystem engine for docker).
Still, I hope the deletion of the other snapshots (9 of them) and reballancing will bring some dragonly performance upgrades 