Request Timeout: Server was too busy to process this request

This happens when a SQL query times out due to not getting a lock. This indicates a bug in the application. Some other job may also be acquiring a lock on a related table, causing the issue. Any recent controller hook or scheduled job added should be reviewed.

One easy way to debug this is to perform the action that triggers it and while it is happening, check the processlist of your site to see which queries are running.

Checking slow queries is also a good idea.