Someone had a problem that the fuel gauge did not say 4/4 when full, but maybe 3/4 while the tank was full. So it either the gauge, the wiring, or the sender's fault.

To check wiring and gauge, disconnect the sender connector and short pins G (sender output) to 31 (gnd).
"G" stands for "Geber", "W" stands for "Warnung", and 31 is ground.

If the problem seems to be the tank sender stuck (most likely) it may need cleaning or at least further investigation.