Thank you @wwe for pointing me into the right direction.
Searching the net for 400 errors and blank pages brought me to this issue: Opening Documents returns an empty screen and a HTTP 400 error · Issue #115 · CollaboraOnline/richdocumentscode · GitHub . Though the case didn’t apply to mine (the server is directly connected to the internet), adding the domain name to ::1 in the hosts file did fix the issue.
Some checks did not succeed but it seems to work for now… thank you everyone for your assistance!
