FYI
@Goodboy428 you terminal comand didn't work since you probably don't have a dev/md1 - 5 partition. by default filesystems get names sad, sdb ... and the partition can be access adding the corresponding partition number e.g. /dev/sda1
Then for my unmountable drive Problem. I launched the command
xfs_repair -L
which deleted the Log file and potentially some data. but the disk came again mountable and works as expected.
Thank you all for the help.
As for the explanation why the parity was still valid even when one disk was missing: The parity check is validating the disks not the filesystems. As long as the disk was accessible and had the correct data the parity check come back valid.
I hope this helps some future guys desperate to recover a unmountable disk.
tschinz's post in Unmountable disk, missing files and docker containers disappered was marked as the answer