Well, other than the usual disclaimer
about putting a database file on a network share like a PST or OST file, I can't say I've ever seen it before. You'd probably want to get a network trace via netsh or netmon or wireshark of the PC attempting to hit the file, and you might even want to get a trace from the server simultaneously as well. There's nothing inherent I can think of that would cause it, other than Windows 7 would be using SMBv2 to the 2008 box(es), whereas XP would be using SMBv1 - that can have an impact depending on the command responses they are seeing (and whether or not offline files caching is enabled for the location).
Again, PSTs and OSTs on the network are a bad thing, and doing so will only end in tears (if not now, eventually).