How to setup two Federated NC with Collabora Document Server | Work on documents inbetween the federation

Nextcloud version (eg, 12.0.2):
Operating system and version (eg, Ubuntu 17.04): Ubuntu Server 18.04 Up to date
Apache or nginx version (eg, Apache 2.4.25): Apache 2.4.25
PHP version (eg, 7.1):

The Setup i want to achieve:

Nextcloud A
Nextcloud B
Both NCs are federated.
I want to create a document on NC A with user X and share it with user y on NC B.
After that we want to work together on this paper with collabora. One Collabora Server connected with NC A and NC B.

What ive achivied right now:
Two NCs A and B. One Collabora Server connected with the collabora App in both NCs. User x can open a document via Collabora and can edit the document. Share a document from NC A as user x to user y at NC B works as well. User y can also create and edit documents on his home NC B. After accepting the shared document the user who created the document can open the document in collabora. The other site get an error telling: Cant load Collabora Online Edition- Please try it later again.

How to set it up so both Users can work on the same document in a Federated NC world.

Is this the first time you’ve seen this error? (Y/N): y

Steps to replicate it:

  1. Two Federated NCs
  2. One Collabora Server
  3. Share a document to the other NC and try to work on the document together

The output of your Nextcloud log in Admin > Logging:

Nothing regarding to the problem. Just telling that is has no internet connection, which is right because the NCs and the Collabora Server are in a private net. Without inet connection.

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):

Nothing as a fresh configfile because there has been not realy done that much. But if needed i will add it!

The output of your Apache/nginx/system log in /var/log/____:

No Errors when trying to edit the Document.

Any ideas welcome. Who has setup such a configuration and can help me to achieve it as well. Thanks.