MacRumors:

A number of Mac apps failed to launch for users over the weekend because of a change to the way Apple certifies apps that have not been bought directly from the Mac App Store.

Several users of apps including Soulver and PDFPen who had downloaded the apps from the developers’ websites all reported immediate crashes on launch. Developers of the apps quickly apologized and said that the issue was down to the apps’ code signing certificates reaching their expiration date.

This is a minor annoyance and the fix is relatively simple.