I have about 10 dashboards to refresh every night. A couple of weeks ago random ones started failing because of a "timeout" issue with the data source. But it's not the same dashboards failing every night and they don't fail on the same table import every time either. If I then manually go in and refresh it myself, there is no issue.
The exact wording of the error is:
"Before the data import for *table name* finished, its data source timed out. Double-check whether that data source can process import queries, and if it can, try again."
The data source I am using is SQL server. The gateway is completely up to date (from limited googling, this was one of the issues), and there is no timeout set on any of my queries, or on the database itself.