Unexpected XFS SB number 0x00000000
Seems like using parted to write a new partition table did the job for me. Used 17.4kB as start and 3000GB as end.
Now being able to mount the partition successfully I ran xfs_growfs. Afterwards, I still had to do an xfs_repair though, because there were a lot of (seemingly) random errors when accessing certain files. Everthing normal now...