Does openapi 3 allow 200 response text/plain &&& 401 response application/json?

Yes, OpenAPI 3 supports this. Media type and schemas can vary per HTTP status code, and it's even possible to define multiple media types with different schemas for the same status code. For example, this is a valid OpenAPI 3 definition:

      responses:
        '200':
          description: Plain text or XML
          content:
            text/plain:
              schema:
                type: string
            application/xml:
              schema:
                ...
      responses:
        '401':
          description: Unauthorized
          content:
            application/json:
              schema:
                ...

The issue you're seeing seems to be a bug or limitation of the Connexion framework. Please follow up with them to troubleshoot the issue.