The format of preferredserver.dat is something like this: 1259063829?server1;server2;server3
The 1259063829 is a time stamp (11:57 GMT today). The time stamp appears to be optional. It's normally there but things seem to work without it.
So what you got is an empty preferred server list, with only the time stamp. One way you'll get an empty list is your client is not in the preferred server's IP address range. Also, when you change preferred server settings, it may take up to a day before clients will use the new settings. Clients that have an existing preferredserver.dat keep using it until a local task that runs once per day deletes it. Try the following experiment: manually delete preferredserver.dat from the client again. Then do a distribution. The client will get a new preferredserver.dat and, unless it has the "wrong" IP address, your preferred server should be listed in it.
Thank you for this clear and precise answer, I searched the forums a long time without success.
After verifying, my address range was wrong.
Thanks a lot.