Mass Update "Assign to" broken

[Romanzote]

Hi, using 4.5.0h with php 5.1.4 mysql 4.1.7 on a W2003 Server IIS&.
All my mass update works. Only the "assign to" does not work.
No error message, just a blank list and no changes in the lead wich should be assigned to another user.
Could anybody help ?

Roman