@vcunat (cau Vlado). The problem is, that the FS is mounted, so the btrfs check will not run. Maybe the last-resort restart option “5 LEDs or more: Boot to rescue shell”? If the main FS can be accessed via the rescue shell, it could be done…
@Weafyr: For me, the reflash from USB worked well (https://www.turris.cz/doc/en/howto/omnia_factory_reset). I lost my LXC containers, but the rest seems to be working fine (backup the /etc from the broken FS and then put back after reflash).
The question is, as I ask in my thread (Error opening journal: Read-only file system), is this a memory problem? Is is worn out? Is it good idea to run LXC with simple web accessed mainly by bots and just a few humans there? Should be the LXC webserver moved out to SSD (how)? These are questions that bothers me now. I have a router with possibility to run LXC container with simple webserver (yay!), but I’m afraid to do it, to not wear the built-in memory…