well, after long nights, the solution now is easy.
Double-check the apache proxy configuration.
Enter the correct paths in loolwsd.xml.in , don’t forget to set the true/false options where the relative/absolute paths are given.
check whether https://office.yourdomain.de/loleaflet/dist/admin/adminSettings.html is working after make run.
Now you know your proxy settings should be correct.
use the URL https://office.your.domain.com:9980 in the rich documents app.