Hello Hartwig,
Unfortunately the log does not help here.
There isn't detailed information mentioned in the logs, it is very hard to address root cause. I would suggest you to raise a ticket to SAP support by providing following information:
1. sapinst.log
2. sapinst_dev.log
3. sapint_loginquirer
4. diagpack.tgz (after executing command "dbmcli -d <dbsid> -u <username>,<password> diag_pack", you will find it from maxdb run directory)
Wish you good luck.