Geared Nema will not turn

My Rostock Delta mini pro came with a mounted gearbox on the extruder motor.
Now, when testing, the motor will only give a smal hick and than is unmovable stuck.

When I push the emergency stop, it will turn free again.

So somewhere it crashes on start. Could it be the gearbox ? The motor ? Any advice ?
I think I will dismount the gear and try again......