This user is disabled due to mapping a network drive because the password does not match up. If their PC signon ID is the same as their AS/400 ID and the password is different then it will diable after trying to map a network drive. Especially if it is set up to map the drive each time the user logs into the PC.
If you go to OPS Nav and Network, then Servers, then TCP/IP and click the screen will show up with all the TCP/IP servers and their status. If you right click on iSeries Netserver, you will go to disabled user IDs and there you will see who is disabled and you can reenable them or do what you have been doing and change the user profile in some way to get it to reset.
This is a problem I see all the time and am always telling the users to make sure their PC/Network passwords are the same as their AS/400 passwords if the IDs are the same.
Also, the AS/400 does not care about upper/lower case.