Reverse proxy problem

Hello,

I've got an Apache instance running on port 29000 and one of the
virutalhosts in this apache should work as reverse proxy, so I
configured the VirutalHost like this:

<VirtualHost *:29000>
ServerName www.website.com
ProxyRequests Off
ProxyPass / http://internalmachine.domain.tld:5959/
ProxyPassReverse / http://internalmachine.domain.tld:5959/
</VirtualHost>

Now the webserver itself running Apache is on an internal machine and as
stated running on port 29000, that port is mapped outside (on the
internet) to port 80 of course. The problem that I now have is that when
someone accesses that virtualhost, the first request works but then it
tries to access www.website.com:29000, yes on port 29000. That sounds
strange, I don't understand why it then do the following requests on
port 29000, this doesn't happen with other normal virtualhosts websites.

Does anyone have an idea what's wrong ??

Btw: I cannot use port 80 because this is already used for another
Apache instance.

Regards