QNAP TS-509 Raid5 "not active"

After a disk "fail" my Raid5 array went down.
I took out the disk, formatted it on my workstation and put it back in. After that all the disks (3x2TB) show status "GOOD" but the Raid volume wont rebuild.
RAID 5 Disk Volume: Drive 1 2 3 Unknown Not active
When I try the built-in recovery i get the following error in the logfiles
[RAID5 Disk Volume: Drive 1 2 3] RAID Recovery failed: Not enough devices to start the array.

QNAP-Support had a quick look and then asked me to return the hardware for repairs. I'd really like to avoid that.

Any help/tips on this??