Have you recently added any plugins? If had, remove the newest and try again.
How to do that?
Go to My computer > C: (I mean the disk where you installed PDN) > Program Files > Paint.Net > Effects.
Remove the latest added plugin (or plugins). If you don't remember which did you last add, View > Details to turn on the Details View and click the Date Modified to sort the files by their modification (addition) date.
Go to My computer > C: (I mean the disk where you installed PDN) > Program Files > Paint.Net > FileTypes.
Do the same thing like in the tip above.