This question is similar to the closed issue:
https://community.quicken.com/discussion/comment/20247571#Comment_20247571 Every time I reconcile an investment account and change the status of a transaction from cleared to not cleared (or vice versa) I get an "invalid date" pop-up box pointing to the transaction I changed (Note: the transaction shows a valid date). Once I click OK the reconciliation finishes as you would expect. I have tried all the suggestions in the earlier port - copying the file, validating the file, ... None of these fix the problem or report an error.
I am running Quicken for Windows R45.13 Build 27.1.45.13 which as far as I know is the latest version. This problem has been around for several months - I can't remember exactly when it started.