This error typically occurs when Server performance needs to be improved to handle the load. Here are some articles that suggest steps you can try:
Optimizing Tableau Server Performance:
Monitoring Tableau Server Performance:
Performance Improvement for Loading Views on Tableau Server:
If you are still unable to resolve the error, I would recommend submitting a ticket to our support team at firstname.lastname@example.org
1. Is this issue ongoing or periodic?
2. Date and Time the issue occurred
3. Server Log Files (See http://www.tableausoftware.com/support/knowledge-base/creating-tableau-server-log-files for instructions on bundling and sending the log files)