Problems connecting to collabora

Hopeful everything seems to be up’n’running - docker, nextcloud, collabora.
The Integration seems to work well, till opening a Document or creating a new document.
The toolbar from collabora is rendered corrctly but no document is shown -> Spinner “connecting” -> Ending up with a “Connection error message”

Using Ubuntu 16.04
Nextcloud 9 (newest Version)

LogFiles are showing the following entries:

[Fri Jul 22 09:48:27.362296 2016] [ssl:info] [pid 25050] (70007)The timeout specified has expired: [client 10.0.0.112:49565] AH01991: SSL input filter read failed.
[Fri Jul 22 09:48:27.407249 2016] [ssl:info] [pid 25048] (70007)The timeout specified has expired: [client 10.0.0.112:49568] AH01991: SSL input filter read failed.
[Fri Jul 22 09:48:29.173620 2016] [ssl:info] [pid 25051] (70007)The timeout specified has expired: [client 10.0.0.112:49567] AH01991: SSL input filter read failed.
[Fri Jul 22 09:48:29.256545 2016] [ssl:info] [pid 25049] (70007)The timeout specified has expired: [client 10.0.0.112:49573] AH01991: SSL input filter read failed.
[Fri Jul 22 09:48:29.256872 2016] [ssl:debug] [pid 25049] ssl_engine_io.c(1016): [client 10.0.0.112:49573] AH02001: Connection closed to child 12 with standard shutdown (server office.racke.org:443)
[Fri Jul 22 09:48:40.923547 2016] [proxy:debug] [pid 25084] proxy_util.c(2175): AH00943: WSS: has released connection for (127.0.0.1)
[Fri Jul 22 09:48:40.923976 2016] [ssl:debug] [pid 25084] ssl_engine_io.c(1016): [client 10.0.0.112:49577] AH02001: Connection closed to child 1 with standard shutdown (server office.racke.org:443)

any ideas?