It's real hard to say without more info. What patch release of 10.4 are you running? Have you opened a case with Tableau Support? Does the error occur when running directly from Tableau Server instead of running from your web app? Are you running low on system resources (i.e. CPU, RAM)?
When looking at the logs, search the logs by the session id to find more detailed info.
Thanks for the pointers. I am able to view some sessionaspect.acquireandreleasesession exception in the VIZQL.log file. I am working to get this submitted to the Tableau. Struck due to some bureaucracy in accessing Tableau customer portal.
Arun, was your issue resolved? any pointers on the solution.
I am facing a similar issue.