Transparent proxy blocks openvpn

We have a net-to-net connection which the users use to connect to a server on port 80.
After the connection is made, everything works just fine. However, sometime in the next day or so, the port 80 stops working. What I mean is, we can still ping the machine etc, but the proxy decides to start blocking port 80 to that server. A restart of the transparent proxy will fix the issue. However, I’d like to know if you guys have an idea how I could stop this from happening short of disabling the transparent proxy…