I just got the same exact message.
I don't know what is causing it but I did upgrade WhatsApp (deleted the old version 2.2343.55 and installed from Mac App Store version 23.23.78) a few hours ago and I wonder if it's related.
Do you recall upgrading anything before getting the alert?
In this case it's safe to click "Open Anyway".
I actually had problems with the Sparkle updater that the non-MAS version of WhatsApp uses to update itself. Several times it showed a dialog to notify me of an available update, but then hide the dialog behind the main window, so that I did not immediately see it (I think sometimes I did not see it for several days). Sometimes, upon trying a manual update, I also got the error that the "updater" was already running. Automatic updates via the MAS hopefully resolves these problems and makes the app less of a hassle, notwithstanding the "ServiceExtension differs from previously opened versions" dialog that I also received (and which lead me here).And since, we here are a bunch of very specific folk (who replaced downloaded whatsapp from app store's one).
How are you? How has been your experience with both the versions? What was your rationale behind this move?
I'll tell you about mine.
I upgraded because I felt that the app store version would be able to keep itself better updated.
and
I wanted whatever thin/thick guardrails App Store might put on apps regarding sandboxing and data access.