Hi !
Same behaviour for me but after a failing disk replacement on raid10 pool and perahps a Rockstor update.
Tried srubbing, balancing and always got “failed to read the system array: -5”. But pool not mounted on startup.
Btrfs Check ending with 0 errors on every 4 disks. With “btrfs device scan” I could manually mount the pool.
After I make a new Rockstor install, deleting previous boot disk only, then import my pool disks and all is OK.
BTRFS is still unstable ! !