CRM 2015: No plugin have been selected from the list. Please select at least one and try again

hi,

i developed an plugin on a CRM 2013 system where I could register it without any Problems.

Then I copied that dll to the production-Server and tried to Register it there (CRM 2015) and I get this error message: "No plugin have been selected from the list. Please select at least one and try again"

I read several Posts but none helped me. I updated my plugin from .net 4 to .net 4.6, the plugin-class is public, tried 2013 and 2015 Registration-Tools in different Versions.

what could be the problem?