If you have standard AS/400 passwords as shipped (i.e. QPWDLVL of ‘0’ or ‘1’ and NOT ‘2’ or ‘3’), then mixed case Windows passwords won’t be recognised. Keep your network password all in lower case. It won’t recognise it otherwise. Once you reset your password, you’ll need to re-enable your Netserver profile.
Also, there a re a limited number of special characters available to you: @ and underscore _, for example.