I am receiving the same error message. It only exists for older files (haven't been able to nail down when the problem started though). I have no problem creating a new InfoPath form and submitting it.
Aside from receiving that error message when submitting, I cannot update the form properties directly on SharePoint either for the historical files. I receive this error message telling me I modified it nearly a year ago:
"The file Assets/20 Montchanin Corporate Center_ Greenville_ DE 19807 - 966.xml has been modified by OFFICE\kcorbe01 on 10 May 2013 15:02:32 -0400."
It's almost like there is a file lock dating back to then? Weird.