Why should they challenge your logs?
If they really want to check they could always take the radar trajectories, as they store them indefinitely. It sounds a bit as if you are trapped by some kind of German "Hauptflugbuchzwang" (obligation for an airfield to have a flight record documentation) and "Reichsjudenfluchtflugleiter" (ground person to record all flight movements to prevent german jews to bring their wealth out of the country)? In todays surveillance world we do not need these stinkin' freaks, we have ADS-B and Mode-S ... ;-).
Btw: I never heard that your personal flight log must be validated by a respective ground record. Do you have any regulation stating this? How can this be done in countries where there are no ground record, or at private strips, or or or ?