New Situation
Once I started testing in logging mode the problem had gone away already. There were some hints to HTTPS certificate issues, but nothing really to blame. Neither with nor without logging the problem could be reproduced after two days of questioning me.

Reproducibility
The problem cannot be confirmed generic on domain level. I can reproduce the effect with keys shipped from my domain, i.e. email addresses @shimps.de, but the issue vanishes when I try to reproduce it with email addresses @gnupg.org as e.g. Werner's address.