... I solved the problem: I had some older already existing license keys - even after I deleted 2 of the old keys saving the new one was still impossible (although the message box "Saved successfully" appeared. But after I deleted all old license keys, it was possible to enter and save the new one.

Hi Joern,
We just tested the license key manager and it was working properly. Can you try to right click on the license key manager and select "Run as Administrator" - that way you'll ensure that the app has rights to write to the registry. Let us know if the problem persists.

Best Regards,
Nevron Support Team

Hi Experts,
after installing the new Nevron version 2018 (NDotNetVisionSetupVS2017.msi) - before I did uninstall my old version 2017 - I tried to enter my new received license key with the license manager. After that, a message box with "License sucessfully saved" appered. So far so good. But when I tried to run a project, the "unlicensed" version text is still in the chart. So I opened the license key manager again - and here are now only the old license keys, the before entered new license key is not listed. And yes, I tried to enter the license key again with admin rights - same behaviour. 
=> How do I enter the license key now?

Thanks for your help,
Best regards,
Joern