Hello
We had similar problem that the password was deactivated for dialog user in SU01 sometimes.
After reading this topic and some testing I found that the password is deactivated when it is reset to initial password in SAP and user before change to productive password logs in to the Portal system first (which is connected to this SAP system) and open some transactional iView there. Then the password deactivates in SAP.
So the solution in our case was that the user after password reset should change it in SAP first and then he can work in Portal.
Maybe this will be helpful for somebody.
Michal