https://.../api/swagger.json : Could not load file or assembly Newtonsoft.Json Version=13.0.0.0

Hey. After removing the Newtonsoft.Json Refrence from all of my Projects in the solution i am a step further.
These Endpoints now working..
https://.../api/openapi/v3.json
https://.../api/swagger.json

and showing me a valid json in browser 🙂

But now i am facing the next problem. The UI still shows the same error.
Clicking on the yellow "validate"-Button at the bottom right corner, gives another
Error:

{"schemaValidationMessages":[{"level":"error","message":"Can't read from file http://.../api/swagger.json"}]}