Sorry, this is about as far as I can go as I don't have a test environment.

The only things I can think of are:
  • Maybe the LDAP server is throwing an error - have you tried querying using the LDAP servers native client tools to make sure that the search works? Are there any log files associated with the LDAP server that you can check?
  • Maybe the data type you have used simply doesn't work with the MS providers? Have you used the same data types as on the standard objects?