Connections over an openvpn tunnel break down after a short period of successful usage

Some ISPs use smaller MTU sizes than the standard 1500, which
causes this problem.
Endian Firewall already gets rid of this problem and limits openvpn packets to 1450. In some cases this may also to huge.
You can manually lower this size:

1. Edit the openvpn template with an editor of your choice:
   nano /etc/openvpn/openvpn.conf.tmpl
2. search the string mssfix 1450
3. replace 1450 with a lower value, for example: 1200
4. restart openvpn by calling:
   restartopenvpn