Not sure where to report bugs - so put it here for reference of other users trying to import user fields:-
(Moodle 1.95 build 20090923, IIS, SQL Server)
- If you try to import a file with reference to a user field that doesn't exist, instead of a sensible error I get taken to a "Page cannot be found" instead of a sensible error.
- If the profile field short name is in upper case, then importing doesn't work. (again, "Page not found error")
Not a problem, as I have a workaround - change all my profile field names to lower case.