WYSE 3040 factory reset not working (error code 315)

Dear forum,

I've got a problem with a newly purchased WYSE 3040 - somehow initial set-up was messed-up.
Now device is booting directly into desktop - as "thinuser" - without any menu or options. 
I tried the "G" key reset mode, which didn't work. As well I tried to re-flash the device via USB stick -> but once image is starting, I'm getting following message "error while checking multiple connected USB (error code: 315)

Except keyboard, mouse and USB stick with new image, nothing else is connected. Tried as well to disconnect keyboard / mouse after choosing USB boot option - but I didn't work.

Any idea how I can unbrick the device???

Thanks in advance