That's  my all-time #1 favorite InfoPath Forms Services error.  Really, what could be more useful and informative than that?

If you've used Forms Services with K2 very much, you've seen this annoying, frustrating, useless error.  Forms Services tells you can't open (or submit) a form because 'some rules were not applied.'  What it doesn't tell you is anything useful, like what rule, what error.

It is possible to enable more logging in Forms Services to put more details into the SharePoint log.  But here's a simple trick you can use to find out more information about the error.  Simply go back into the InfoPath integration wizard and change the option to deploy the form using the InfoPath client.  When you get this error in the InfoPath client it gives  you a "Details" button that will give you some sort of vital clue about the error.

When I'm creating a process that uses Forms Services InfoPath integration with K2, I always leave the client option checked during the early rounds of testing.  Later once I'm sure the form is working, I switch back to Forms Services.