-5000 errors when some users attempt AFP login

Starting sometime today (AFAICT), one of our File Servers has started giving out -5000 errors (this is normally a permissions error, right?) when two specific users attempt to authenticate. Everybody else can authenticate fine (including when using their workstations) and, in fact, they can authenticate successfully (and mount volumes) from another AFP server. Both AFP servers are bound to OD. The AFP server they're experiencing difficulties with is running Mac OS X 10.4.8 Server and the other is running Mac OS X 10.3.9 Server. I'm not seeing any errors in either /Library/Logs/AppleFileService/AppleFileServiceError.log or /Library/Logs/AppleFileService/AppleFileServiceAccess.log Any suggestions as to why this might be happening or where to start looking? Could it be an OD issue? Thanks in advance.