Nikon SB 910 Zoom Err in display

[MEPER]

I just got my 2nd hand SB 910 flash. It looks like new but I constantly get "Zoom Err" in display.
I can read camera ISO and Aperture in display.
I am quite sure the "power zoom" is activated.
It comes up with this error both with flash on and off the camera hot shoe (I have tried both D5200 and Z50).
Flash can detect that it is a DX camera (in menu it is set to switch automatic).
In the beginning I could see and hear the zoom head was moving. Now it just stays at the front all the time and a "faint" sound until the "Err" shows up in display.
My own conclusion is that the flash has a fault and I should try to send it back to the seller.
It fires and seems to work ok at the widest setting (when the lamp is at the front this must be widest setting).
I have tried to see if I could set zoom to M but it does not react on Function key 1 (I have the understanding that I should hold F1 down and then turn the wheel when power zoom is set to M).
I could have misunderstood something. Therefor this topic :-)
I use Eneloop batteries but I have also tried fresh Alkalines.   

[MEPER]

Update:
Sometimes the flash zoom can still move the zoom head all the way from back to front.
I wonder if it is something with the "diffuser" contact that it sometimes think it is in use or could be mechanical obstruction.

[MEPER]

I watched a video where I could see/hear how it normally sounds.
It moves the head quite fast to the back and to the front without strange sound.
When my does it there comes a "rattling" sound when it is at the back and once again when it is at the front and then the "Err" shows up.
When I turn on and the head does not move then if I shake the flash it moves so conclusion is that something mechanical is bad.
Maybe a shipping damage. It was packed very well.