This user has likely been set with a Rehab Management user id that is lower case. The RM user id should be entered in Case Manager as upper case.
To fix, this, go to the User Profile in CPM Administration Tools and reset the password. You can keep the same password, but just enter it again. The user name will then change to all caps and the user should be able to access the system.
Do not change the RM user name in Case Manager as this may create another user in Client Payroll Manager.