Urbackupserver behind nat and apache reverse proxy

Urbackub is a very nice application I’m trying to explore.
I have had some problems connecting from internet clients using the default port 55415 due to external firewalls.
I would like to use port 80 instead. So to test it i forward port 80 on my router to the urbackup server which work just fine.
My next step is to forward port 80 on my router to my “frontend” webserver(Qnap) which is using this directive in virtual hosts file:
Trying to send the data to the urbackup server (This works with another backup application but off course might not here, though it would be nice)

<VirtualHost *:80>
ServerName urbackupservername.mydomain
ServerAlias urbackupservername.mydomain
ProxyRequests off
ProxyPreserveHost off

ProxyPass http://localipofurbackupserver
ProxyPassReverse http://localipofurbackupserver
SetOutputFilter proxy-html
RequestHeader unset Accept-Encoding
Order deny,allow
Allow from all

The client status show connected to internetserver, but the urbackup interface display “no” for connected.
And backups fail instantly - since connection is probably broken.

So somehow I am creating a “glitch” in the communication, am I on a wrong track here?

Any answer is greatly appreciated

Kind regards