If you are trying to call a published model via the Domino Model API tester page and run into problems, please check the instance logs for issues. If you see something similar to the below warning you may be running into issue with your cookie size:
[WARNING] unable to add HTTP_COOKIE=dominoSession=ae72fd0d72e7;
to uwsgi packet, consider increasing buffer size
To resolve this issue please try clearing out the cookies in your browser before trying the model again. If you continue to see an error message similar to the above, please contact support.