Collabora shows empty window/contents for every file; owneruid error

I’ve set up Collabora Online on my NextCloud instance and it doesn’t work: it will show the Collabora CODE window, but the contents will be blank, no matter which document I open. Scrrenshot:

(This also happens when I open a document manually in Collabora, like so: )

I’ve set up everything yesterday, according to the instructions. Server info: Arch Linux with package nextcloud-app-richdocuments 1.12.33-1, installed yesterday - Collabora Code is the only container in a freshly set-up Docker instance, downloaded yesterday. Server is running apache 2.4.27-1 and php-apache 7.1.7-2. Only speciality I have is that I’ve opted to use the same DNS for the Nextcloud instance as for the Collabora server - but I wouldn’t understand why that would be a problem. I just see no reason to get another DNS for something that is essentially just a feature of NextCloud. And the proxy does seem to be working, I do see the window for Collabora - just nothing appears within it, there is no error message.

The richdocuments app however produces an error in the Nextcloud log when I open a file with Collabora: "Error PHP Undefined variable: owneruid at /usr/share/webapps/nextcloud/apps/richdocuments/lib/TokenManager.php#88 2017-07-28T13:20:40+0200"
Only log entry in Docker when I open a file is a warning: “wsd-00027-00035 11:20:42.162712 [ websrv_poll ] WRN WOPI host did not pass optional access_token_ttl| wsd/FileServer.cpp:355”

How do I fix this? Have I made an error in my setup?