I set up a dude as log server, this server work fine but the last week stopped working.
If change default port 514 with 520 ( and Rb client log ) dude accept logs.

Now I don't have any rules in firewall:
/ip firewall filter export
# sep/16/2012 17:09:42 by RouterOS 5.20
# software id = xxxx-xxxx
#

Any idea?