SSRS - Sys.webforms.pagerequestmanagerservererrorexception: An unknown error occurred while processing the request on the server. The status code returned form the server was: 500

Background:
The report would run if a small range was entered. (4 at a time)
It would only throw this error if no filters were used.
The report used to work in the past, and suddenly one day starting giving the error.
The report was calling a subreport that would be called multiple times

Cause:
The view that was being used as the source for the SSRS report had multiple unions, partitions and calculations in the logic.
A stored procedure was created to output the view to a table first.
The SSRS was then connected to that table, and everything worked fine.

Conclusion:
SSRS cannot handle extremely complex views as data sources.
You must output the results to a table first before handing to SSRS.