Dell 2155cdn, scan not possible, error message 'scanner communication not possible'

Hello

for years, our 2155 cdn worked fine, but since some months, scanning is no longer possible.

Opening Dell Scan Center and pressing the button to initiate a scan results in 2 error messages:

1. "Verbindung zum Scanner konnte nicht hergestellt werden. Scanjob erneut durchführen."

Translation: Connection to the scanner could not be established. Restart scan job.

A few seconds after confirming the message, it reappears again, ~20 times in total.

2. After ~20 times of the first error message, the second appears 1 time:

"Kommunikation mit dem Scanner nicht möglich."

Translation: Communication with the scanner is not possible.

The 2155 cdn is connected via LAN. Printing works fine. Making copies on the printer itself works fine, too.

So far I have tried the following to solve the problem:

De- & reinstalling the complete Dell Software on Machine1 (Win7 Ultimate SP1) -> same symptom

Installing the Dell Printer Software from the original disc on fresh Machine2 (Win 8.1) -> same symptom

Trying scan job from Machine3 (Win7 Pro SP1; System was not touched in many months and scanning was formerly possible from there) -> same symptom

Restarting the 2155 -> same symptom

Checking 2155 configuration and trying to change and rechange scanner settings -> same symptom

Connecting the 2155 via WLAN, instead of LAN -> same symptom

I would really appreciate if somebody could provide some help on that issue.

Many thanks in advance.