Do you have additional details on the error you are encountering and what you have tried?
I found out that the errors was generated when one of the VizQL servers went down.
You will see the following errors:
exception: Instance 'vizqlserver#1' does not exist in the specified Category..
exception: Instance 'tabspawn#16' does not exist in the specified Category..
When I ran the test with less concurrent users, I don't see these errors.