When you press the validate now button the following error may occur at some point during the validation of the review screen rows:
This occurs when the following conditions are met:
1. Importacular is set to create a query of update exceptions
2. There is an exception with a record that is marked as update.
3. You are validating the rows (as opposed to actually importing them).
This error was fixed in version 2.5 patch 1 released for self-hosted organisations on 13th June 2017.
This error was fixed for Blackbaud hosted organisations on 14th November 2017
Workarounds:
If you do not want to import the records into RE yet and want to do a validation run over the records first, then ensure that the create update exception query is not selected. If you still want a query of these exceptions then you will need to import those that do not except.