Quote Originally Posted by AAEBHolding View Post
@df: You are right, but this is not the problem.
@volume full: This is not the problem and does not solve my issue.

I may have been not accurate enough: The root filesystem is totally damaged. When I start the server the 1st time [from a 3 weeks old backup] so I get after a while the above error messages.
When I try to start the server then the lot of error messages appear and the server becomes unavailable. I found also this post where the same error messages are posted.

Please note that the system is referring with the message 'kernel BUG at ../fs/btrfs/relocation.c:1449!' to a bug in the kernel in the 1st line of the dump!

Again: The server is idle, this means, apart from the normal services, nothing special is running. Suddenly, the error messages in the 1st post appear in the console and then the VM is broken!

It looks to me like the kernel got a very serious bug in one of the previous updates.
At GRUB, can you select advanced boot options and boot to an earlier snapshot?