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Problem solved with the help of the Support Ninjas :)

Short answer: resetting the Omnifocus preferences by deleting the preference file.

Long answer:
Quit OmniFocus, then open a Finder window. In the menubar, select Go>Go To Folder... and enter "~/Library/Preferences." Move the file com.omnigroup.OmniFocus.plist to the Trash. Relaunch OmniFocus.