Why are Activity Imports failing with error message "source contact id not valid"?
The issue we were having was with two fields in the import field mapping having the same name.

Amending the field labels through Admin>custom data to allowed us to able to link the data to the right location.

-As I said both fields shared values and labels so it's not clear why this was a problem. Or at least, became a problem between this month's import and last months.