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.