I found the solution by my self.
After several debugging, trial-and-error session and minimal setup's I could solve the issue.
I found the final hint here:
https://fusionauth.io/docs/v1/tech/oauth/endpoints/#json-web-key-set-jwks
FusionAuth did not create asymetric key's in default, so I have to create a new asymmetric key that can be validated at the JWK endpoint by apache2.
Zero