Web Forms for Marketers: an exception: has occured while trying to execute an action.

We've had this error it seems a couple of times now whilst upgrading from 2.2 to 2.3. Thankfully Sitecore support were on the ball as usual and it's an easy fix.

During the upgrade the /sitecore/system/Modules/Web Forms for Marketers/Settings/System/System Action folder can get a bit out of sync. To fix just make sure your version looks like the image below: