None of this makes any sense, so its best you just start from the beggining and recheck everything.


Reported fault:
Grey background on scans, only when scanning from flatbed.
This does not occur on ADF scans.

Conclusion: Scan Assy works perfectly fine when stationary. Image processing is fine.

Possibly problems left:

Imaging processing issue exclusively with flatbed scans due to faulty firmware/board.
Calibration problem/Configuration problem
Cabling on scanner is damaged causing issues when scanner moves, such as poor LEd illumination.
Light leakage from flatbed
Issue with flatbed scan glass.
Issue with white sheet opposite flatbed scan glass.

Following the thread so far, you've already confirmed its none of the above. This would mean that it is working. There is literally nothing left to check in the whole machine afaik for failure, unless we're talking some bizzarre 1 in a billion chance that there is a power or other unique issue only occurring when the scan head moves (unlikely).