spreedme stays at 'initializing'

[hj-beckers]

I'm trying to use spreedme according to https://github.com/strukturag/nextcloud-spreedme:

operating system is openSUSE Leap 15.0 (yes its beta, but nextcloud needs php > 5.4, php ist 7.2)
I'm behind a firewall;
my nextcloud is reachable via "https:";
WebRTC is generated;
spreed-webrtc-server is listening with this configuration on 127.0.0.1:8080;
apache is configured according to https://github.com/strukturag/nextcloud-spreedme/blob/master/doc/example-config-apache.md and listens on 80 and 443

Whenever I click the spreedme icon in nextcloud, all I get is a greyish background and the string "Initialisiere" (= "Initializing") as header. The icons for chat, microphone, camera and configuration show up, room is "standard", but no connections. When i select the configuration button, on the right side of "profile" is shown "unreleased 1542809242".

That number is also shown in apaches accesslog:
"my.proxy.I.P - - [27/Apr/2018:07:50:11 +0200] "GET /webrtc/static/ver=1524807628/js/main.js HTTP/1.1" 200 2963 "-" "Mozilla/5.0 (Windows NT 6.1; WOW64; rv:52.0) Gecko/20100101 Firefox/52.0"

The logs of nextcloud and spreed-webrtc-server show nothing.