OpenVPN not working properly after switching to 2.0

Hello,

I had installed two new virtual machines with pfsense 2.0, with the same settings as my older ones.
Between them, there is a TCP OpenVPN tunnel through a proxy, and all traffic from that site is routed through the tunnel.
On my old config based on 1.2.3, I had no problems, it did not require any special tweaks on the MTU.
When using 2.0, the tunnel gets up just fine, samba traffic works ok at 1MB/s between sites but when trying to browse the web the tunnel breaks.
I also tried pinging, ping -l 1376 crashes the tunnel, -l 1375 is ok.
Now, I have tried different settings for the tun-mtu: tun-mtu 1300 is ok for ping -l 1376, but web traffic crashes the tunnel. Also, ping -l 5000 crashes the tunnel too.
Trying to set MTU 1400 on the network interfaces, web browsing is ok for 90% of the sites - gmail, yahoo, msn.com are not working, ping -l 5000 also crashes the tunnel.

Any clues?