Click here for a post containing a discussion of that error.
Given that, I would start by testing
without the iRule and
with the http profile applied to see if it has something to do with LTM's interpretation of the state of HTTP events for that connection. If the problem persists with the profile but not without, contact Support to open a case to review, then once that's resolved, put the iRule back & test again.
If http profile doesn't seem to be the culprit, post back and maybe somebody smarter than I will be able to help...
If you end up finding the resolution elsewhere, it would be great if you could post back & clue us in for future reference.
Good luck!
/deb