ENDIAN Firewall - VPN connection hangs randomly

Our recently installed OpenVPN server works fine "from time to time".
When it works fine it does so either from Windows client (Endian program) or Linux client (openvpn package in Ubuntu).

Every two or three days it's impossible to get a connection to it from the outside (either from a Windows client or Linux client).
The messages I receive in a Linux client (in console) are the next:

Sat Nov 27 19:29:04 2010 WARNING: No server certificate verification method has been enabled. See <<openvpn web>>#mitm for more info.
Sat Nov 27 19:29:04 2010 NOTE: OpenVPN 2.1 requires '--script-security 2' or higher to call user-defined scripts or executables
Sat Nov 27 19:29:04 2010 LZO compression initialized
Sat Nov 27 19:29:04 2010 UDPv4 link local: [undef]
Sat Nov 27 19:29:04 2010 UDPv4 link remote: [AF_INET]xx.xx.xx.xx:1194
Sat Nov 27 19:30:04 2010 TLS Error: TLS key negotiation failed to occur within 60 seconds (check your network connectivity)
Sat Nov 27 19:30:04 2010 TLS Error: TLS handshake failed
Sat Nov 27 19:30:04 2010 SIGUSR1[soft,tls-error] received, process restarting

These messages repeat indefinitely.

When this occurs sometimes I can connect if I try again SOME HOURS LATER. If I can't achieve to do it I have to wait to the next day.

Any idea?
Thank you in advance.