It is probably the fact that PST's are not supported on network drives. The symptoms you are experiencing seem in line with that:
Personal folder files are unsupported over a LAN or over a WAN link http://support.microsoft.com/kb/297019/en-us
Jeff Pitsch Microsoft MVP - Terminal Server
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Though not specifically a Term Server issue, I thought some smart person here could help.
Our users run either Outlook XP or Outlook 2003 on Windows 2000 and Windows XP PCs'. We currently are running Exchange 2000. The users keep PST files out on their network shares, which reside on Windows 2003 Server Clusters (HP DL 585) and the disks are on a SCSI Fiber Attached EMC Clarion 300 CX SAN. The users will be in Outlook and when they try to get to a folder is a PST they get "The set of folders could not be opened. The server is not available. Contact your administrator if this condition persists". And if they try to move something to or from the PST they get "Can't move items The file * \\servername\filename.pst* could not be accessed" If they click on one of the folders in the PST files a few times it may eventually open, or if they go to Explorer and click on the file from inside there it may. It seems that if they have the file on a SAN based drive it is much more susceptible to this. I know that MS does not support PST files on network drives, but this has been working fine for years, until we moved from fiber attached "local" disks and Windows 2000 to the Windows 2003 / EMC combo. Any ideas?