We ran into a problem with one of our SQL Servers running in Fiber mode choking on some XML functionality. It turns out that Fiber mode should not have been turned on in the first place...
Fibers work best when the server has multiple CPUs and a relatively low user to CPU ratio. For example, on an Enterprise installation with 32 CPUs and 250 users, you may see a noticeable performance boost with fibers. But when you have eight CPUs and 5000 users, you may see performance decrease with fibers.
http://www.microsoft.com/technet/prodtechnol/sql/2000/books/c02ppcsq.mspx
