Dropped packets on WAN interface when uploading from VPN

[GLR]

Hello,

I've something strange in my ALIX 2C3 running pfSense 1.2.2 : When I upload a file (through FTP, Samba, SFTP…) from a remote OpenVPN host, many packets go to the WAN interface (typically almost 200 packets/sec. for a 2,2 Mbps traffic) from the remote public IP to the pfSense public IP and are therefore dropped.

A packet capture trace shows UDP fragments :

 <remote public="" ip="">	 <pfsense public="" ip="">IP	Fragmented IP protocol (proto=UDP 0x11, off=1280, ID=1dd0)</pfsense></remote>

OpenVPN connections are established over UDP.
I have LAN to LAN and standalone PC to LAN tunnels.
Else everything seems to works fine with OpenVPN.

Does anyone know from where it could come from ?