From this post, here's a way to disable/enable parallel querying on the Tableau Server:

 

Here is the cmd script to disable parallel queries for Tableau server 9:

 

tabadmin stop
tabadmin set native_api.connection.limit.sqlserver 1
tabadmin config tabadmin start

Here is the cmd script to re-enable parallel queries if the problem is solved:

tabadmin stop
tabadmin set native_api.connection.limit.sqlserver -d
tabadmin config tabadmin start