Documents not opening, error in browser


i have installed NC 13 and Collabora (with docker) on my machine (Ubuntu 16.04, NGinx).
Clicking on a document results in an error (german message):

Die Verbindung zu wss://cloud.mydomain.tld/lool/https%3A%2F%2Fcloud.mydomain.tld%2Fapps%2Frichdocuments%2Fwopi%2Ffiles%2F55232_ochrnlt4rqmp%3Faccess_token%3D6IjXkZdXTFtLrlu9UFD7Mej821kBZa1B%26access_token_ttl%3D0%26permission%3Dedit/ws?WOPISrc=https%3A%2F%2Fcloud.mydomain.tld%2Fapps%2Frichdocuments%2Fwopi%2Ffiles%2F55232_ochrnlt4rqmp&compat=/ws wurde unterbrochen, während die Seite geladen wurde.

(saying: connection to wss://… was interrupted during loading the page)

Reading the forum threads and googling around brought no help to me.

Can someone help me please?

Thank you very much!

Does anyone know how to solve this problem?
I would really like to get this working.