This is an example config for a site that needs uploads:

   Service "94.124.89.211_80__wepaintyou.massmovement.nl"
      HeadRequire "Host: wepaintyou.massmovement.nl"
      BackEnd
         Address 91.199.219.83
         Port 80
         Priority 5
         Timeout 300
      End
   End

Is it possible that "Timeout 300" is only used for the connection
itself but individual requests have a different timeout ? Also,
please note that uploads using Flash-sites nearly everytime succeed
and it seems that https-uploads succeed as well...

According to the docs the Timeout is the time pound waits for the
*response* from the backend. This should start counting as soon your
request (upload) is completed.

Did you watch the connection using tcpdump? Does the data stop flowing?
Does pound kill the connection, regardless receiving data from the
client?

In the attachement you will find the tcp stream for the POST request. As i see it, this is just a plan POST request that is reset with a FIN/ACK. But why the RST's ? The site went to production so i could not test it as it was but instead i had to create an extra alias as you can see in the dump (marcustest.massmovement.nl). Points to the same virtualhost in Apache.

I hope someone can help me solving this problem.