Not sure which version you're using, but there were some fixes added in IC6.1.5 to handle the keyword arguments properly on the delay function.
Other than that, hard to know without seeing the data. Maybe logging a service request would be your best bet.
If you used the calculator from IC615 to build the expression, it should work - provided you've entered the fields properly.
Can you post the expression, and maybe a picture (save an image from ViVA and then upload it via the Options tab) to show what you're trying to measure?
That said, IC615 had a major change in quite a few parts of the tools. The subversion you are using is the Base release, which came out 2 years ago - there have been a lot of fixes and enhancements in the meantime and so I would strongly recommend moving to the latest hotfix of IC615.