If an expired evaluation license is on your system (either from evaluation or training), it can cause problems with Muk3D. Catapult will keep flashing warning messages that the licenses aren't valid and eventually it will shut Muk3D down.
There are three ways to remove a license.
Through Catapult
Run the command Licensing/Remove evaluation license. This will bring up a dialog listing any evaluation licenses installed on the computer.
Check the box in the Remove column for any licenses that you want to remove. Once you hit OK, it will prompt you several times to check you really want to do this. Evaluation licenses can't be reinstalled, so if you remove it its gone forever.
Through CodeMeter Control Center
CodeMeter Control Center shows the CodeMeter licenses that are installed. A license can be removed by clicking the Remove license button. However on many systems this license is disabled, so this method may not be applicable.
Through the CodeMeter Command Prompt
This is the last resort way of removing a license since it involves the MS-DOS command prompt. Open the CodeMeter Command Prompt by going to Start/CodeMeter/CodeMeter Command Prompt.
The command is:
cmu32 --delete-cmact-license --serial XXX-XXXXXXXX
Where the XXX-XXXXXXXX is the serial number of the license, which can be copied from the CodeMeter Control Center license list. Also note that there are double-dashes before delete and serial.