SSRS - An error has occurred during report processing. (rsProcessingAborted) The processing of Parent for tablix 'Tablix1' cannot be performed. Cannot compare data of types System.String and System.DateTime. Please check the data type returned by the Parent. (rsComparisonTypeError)

So you get the above error when generating a report that usually works.

This is because there is a conflict on your tablix group.

For mine the issue was resolved when i removed the Recursive Parent from my group on the Tablix1 tablix.

Error in SSRS

Group Properties on Tablix, Remove the Recursive Parent