Hello Rainer,
You didn't find a bug in the repo. The editor buttons work via JavaScript editor.js.
I'm having déjà vu: ownership, file, and directory permissions

.
I just visited your two known instances and left you a comment on each. The buttons work for me. However, these two instances show me the
FlatPress fp-1.5.dev signature.
Or do you have another instance with the
FlatPress fp-1.5 RC1 signature where I can reproduce the problem, or is it a local instance that is not online?
It may be helpful to clear your browser cache and disable third-party browser add-ons.
A quick reminder: There is currently no official version 1.5, or 1.5.1.
RC1 stands for Release Candidate 1. RC + number or Beta + number = test phase.
If you have any problems or bugs you've found with a developer (dev) or release candidate (RC) version, then your post is
better placed here.
Best regards,
Frank