But because it's a brand new disk I think I can exclude those causes...
Can you? New never means its fine.
You probably don't want to stay using 126.96.36.1993 it could very likely be that the failed drive never really had a problem.
This applies to RAID 1,5 and 10
Now as to why its not rebuilding:
The drive might not be fine run SeaTools for a long Test.
Then try Erase Drive with Zero ALL
If this does not work upgrade the Firmware on the other drives BUT this might corrupt the drives.
I think the new drive should be ok because I tested it with Seatools (short and long test without errors) and made a full erase which should be even more like a "low level format" than the Zero All...
Meanwhile I downdated the raid drivers - which are 188.8.131.529 now. The Matrix Storage Console stayed at 184.108.40.2063 - I couldn't find software elder than the actual:(
Btw. I'm sure that the old HD was defective. I also tested it with Seatools an both long and short test reported errors and the program hung.
Next is to update the firmware on the other disks...