Dear Marcel Gerhard,
when using Cem2User, please keep in mind you need to define all the functions (unlike most other places in ATENA, where you only need to change the generated properties if you have some good information; in this case, the "predefined" functions are just some examples), and make sure they build a consitant model.
In general, we recommend to see ATENA Troubleshooting, 2.2.4 I want to use the user-defined stress-strain law of concrete to replace that used in ATENA program. How can I do it? and the there referred spreadsheet before you start working with this material model.