Collabora doesn't work behind a Firewall

Hello Everybody

I set up my Nextcloud 10 on Ubuntu 16.04 with Apache (Virtual Appliance of Nextcloud).

When i try to edit a file, from a computer which is behind the Firewall of my company there comes the error:
We are sorry, this is an unexpected connection error.
Please try again.

But when i try to edit a file from a computer which is directly connected to the internet without a firewall, i can edit the file.

So my first thougth was that it only can be the Firewall which is blocking a port. But in the logs of the Firewall there isn’t any packet blocked to the Nextcloud server.

Has anyone an idea or had this problem with his own installation?

Thanks for help!

did you try that? Collabora and issues with cloudflare in a restricted environment
I just updated my workaround, because that was pretty crappy, now its better.

I’ve got the error. The Problem was the SSL-Encryption of our Firewall. I had to disable it for my cloud and it worked fine!
Thanks a lot!