I use Maintenance app to do some checks and cache cleanup from time to time. Today I got a message that my disk should be checked and repaired while in recovery.
So I did and I was surprised to find out this error while checking/repairing /:
snap_metadata_val object: invalid extentref_tree_oid
Apparently this is related to local snapshots of time machine and one of them is in weird state indeed:
fmbpro:~ florian$ tmutil listlocalsnapshots / com.apple.TimeMachine.2019-08-19-210124 (dataless) com.apple.TimeMachine.2019-08-19-233007
So I deleted the "dateless" snapshot:
fmbpro:~ florian$ tmutil deletelocalsnapshots 2019-08-19-210124 Deleted local snapshot '2019-08-19-210124' fmbpro:~ florian$ tmutil listlocalsnapshots / com.apple.TimeMachine.2019-08-19-233007
Amazingly, the error was gone while checking/repairing the main drive.