Deleted an account used for TimeMachine and my RN is now in ReadOnly Mode...

Thanks for your answer. I sent you a private message with the link to the logs.

I hope you'll dig something useful. :)

Still no idea?

I tried a scrub, nothing is fixed.

dmesg shows :

[  105.003830] BTRFS error (device md127): parent transid verify failed on 718766080 wanted 271645 found 322138
[  105.011617] BTRFS error (device md127): parent transid verify failed on 718766080 wanted 271645 found 322138
[  105.011628] BTRFS warning (device md127): Skipping commit of aborted transaction.
[  105.011637] BTRFS: error (device md127) in cleanup_transaction:1864: errno=-5 IO failure
[  105.011641] BTRFS info (device md127): forced readonly
[  105.011645] BTRFS: error (device md127) in btrfs_drop_snapshot:9412: errno=-5 IO failure
[  105.011749] BTRFS info (device md127): delayed_refs has NO entry

And btrfs check --force /dev/md127

WARNING: filesystem mounted, continuing because of --force
Checking filesystem on /dev/md127
UUID: 9d4171ad-fe40-45a0-9013-12e3db1a26fa
checking extents
checking free space cache
checking fs roots
checking csums
checking root refs
checking quota groups
qgroup-verify.c:538: account_all_refs: BUG_ON `ref->num_bytes != num_bytes` triggered, value 1
btrfs[0x42ad55]
btrfs[0x42ad74]
btrfs[0x42ae39]
btrfs(qgroup_verify_all+0x448)[0x42c0bb]
btrfs(cmd_check+0x25c8)[0x44e842]
btrfs(main+0x13d)[0x40c647]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf5)[0x7fd2ab413b45]
btrfs[0x40c67b]
Aborted (core dumped)

I had another error before my last reboot, related to qgroups too...

My last question if someone is still around: what is the safest way to start fresh? Factory default? I guess destroying the volume would not be enough to be sure all is back to "normal"? Thanks if you have informed inputs. :)