Hi Ali,
No, I haven't experienced this myself. I'm using Firefox.
Perhaps it's something to do with the Stumbleupon toolbar? (Although you say it doesn't work when submitting via the SU site itself either... hmmm. Okay, I'm stumped!).
I assume it used to work? Have you changed anything since then (installed a new program, for example?) - anything different about your computer?
Maybe rebooting it will help (my husband, who works in IT, often jokes that computers sometimes just need a re-boot to the head
)!