Thanks to an anonymous comment, I have found out that Adobe has released a hack to fix the "Error opening URL" error in CS5.
It was determined that FileVault was indeed causing this issue. Turning it off was the easy fix, but for those of us who can't do that, we were stuck without CS5. Adobe didn't release a patch, but did release a file to replace in the Commons folder. See link for instructions.
Seems to have worked for me, and that makes me somewhat happy.
Well finally, over a year after shooting, my episode is going to air on Monday! My brother Ryder and I saw a pre-release DVD yesterday, and were really happy with how it turned out!
For those who think reality TV is a sham, everything you see on the episode actually did happen. Of course a lot was cut out due to taking 2 days worth of film, and putting it into 45 minutes, but all the good parts are in there.
Hopefully everyone enjoys it!
...In other words, a Floating Point Error, or so my web research says.
Apparently, in all coding languages (not just Flash), a decimal does not have an exact representation in binary. So, for example, 5.100000000000000001 is seen as close enough to 5.1. This is fine when doing other things, such as moving a Movie Clip to an x position, but if you are coding something such as a calculator when you need to display the exact number, it can be frustrating.
I found a method online that corrects this by comparing a few numbers, and estimating. Seems to do the trick in my case.