are there any other ideas which is the cause for this behavior?

thanks
Norbert

Only idea I can get with infos is to try and disable firewall (just for tests) and check if this fixes the issue.
If so, then possibly it is a firewall configuration that should be changed, or possibly add your projeqtor instance to trusted sites.

i have both tested (without Firewall and trusted sites). The Button is not working.
I think its a Problem with my Server Installation (because on your site with my browser the button is working)
maybe we can increase the debug Level to see more Messages?

i get the following error
unknown error
Datei: ckeditor.js, Zeile: 101, Spalte: 478

After updating i get the following messages (log file)

2018-08-28 13:52:44.827 ===== TRACE ===== DATABASE UPDATE COMPLETED TO VERSION V7.1.5
2018-08-28 13:52:44.827 ===== TRACE =====
2018-08-28 13:52:44.827 ===== TRACE ===== =====================================
2018-08-28 13:52:44.827 ===== TRACE =====
2018-08-28 13:52:44.828 ===== TRACE ===== === SCREEN CUSTOMIZATION ISSUE =====================================
2018-08-28 13:52:44.828 ===== TRACE ===== Einige Bildschirmanpassungen sind nicht mehr konsistent mit der aktuellen Version :
some screen customization is not consistent to the actual version
2018-08-28 13:52:44.829 ===== TRACE ===== - Besprechung
meeting
2018-08-28 13:52:44.862 ===== TRACE ===== Bildschirmanpassungen wurden automatisch korrigiert 😉
screen adjustments are automatically corrected

strange, or?

this means

Ho yes it is very stange.
Unfortunately I can't read the elvish language, so cannot interprete the summoning spell.

😛

Well in fact the message is normal.
It is the screen customisation that tries and fix some fake issue on Meeting object.

We're working on this, it's no real issue.