I just had a similar situation where I was sharing an edit for preview and the person kept getting asked for a decryption key. We tried to figure out if I uploaded to Mega differently, or shared it differently, or jdownloader weirdness since I had not encountered this error sharing anything before.
In this case, the culprit was Safari. When the person used Chrome instead of Safari, it downloaded fine. I have no real idea what the problem was, but I wanted to post the fix here in case it helps anybody in the future.
FWIW, the decryption key is just your password so if you really trust the person you're sharing with (and/or don't have anything important on your Mega account) and none of these solutions work, you can at least give them the key if you really want to.