Shouldn't be. Are you sure that Kerberos is working?
On the client, type klist in the Terminal right after logging in. You should have a TGT from the KDC responsible for the network. If you don't, or it is invalid, stop there, that means you have something going on with your KDC itself. If you do, then login to the AFP server. Run klist again and confirm that you have a service ticket from the AFP server. If you don't, again, that could be your issue.
If you don't get that AFP ticket, ssh into the AFP server and run sudo klist -kt to dump out the service principals for the server itself that are stored in the keytab file. If that file is invalid, missing or just generally gunked up, you have an issue.