I had the same problem of the rule being activated but no task being created in OmniFocus.
For me the problem was I had added
myemail+omnifocus@gmail.com as a contact in my address book and given it the name OmniFocus. When I deleted this contact it worked.
As a side note for support, when I applied the rule to existing messages in my inbox it recognized the email address even when the contact name was listed as OmniFocus.
Also, I was and I am still getting the MailTags error when I try and create a Mail rule in OmniFocus. It seems to only be triggered when I change the "Archived processed messages in" setting. The work around is to add the "move to folder" yourself in the mail rule.