SharePoint 2016 Publishing - Upload document stays pending

Hi all!

I have a Sophos UTM 9 virtual appliance providing Reverse Proxy functionality to publish SharePoint 2016.
All the SharePoint sites are externally published and approachable on https.
The SharePoint environment is internally published on http and externally I use the SSL Offloading method provided by Sophos.
I also have a legitimate COMODO public certificate for my published URLS. In short: everything works correctly externally.

[External]: However, when I upload a document to a library, the document won't be uploaded and stays in the pending mode.

So my next move was to make sure if SharePoint itself was functioning correctly. I tested internally, and this worked like a charm.
Conclusion I make: It has something to do with the Sophos UTM.

I also checked the Virtual Webserver config:

And the Site Path Route:

Logging says:

Is there somebody who could help me out with this issue?
Many thanx in advance!