Not german, but my father always taught us that you must always be early so you can be on-time, and that unless the host has made it clear arrival time was flexible one must be on-time.

We have relatives who usually leave their home at the declared start-time (so they're at best half an hour late, usually an hour or two), they're by far the most disliked members of my extended family for this an other reasons. Arriving late is disrespectful of both host and other guests, and is emblematic of and perfectly in line with their character.