Quicktime cannot connect to YouTube

I am trying to share a video created in Quicktime with Youtube. My Youtube username and password are saved both in Safari/Keychain and 1Password.

I've pasted both username and password into a text file, to make sure I'm not making a typo error.

When I log out of YouTube in Safari, then back in again, the credentials work.

When I try to share a movie in Quicktime, using the same credentials, I get the (unhelpful) message:

"The YouTube name or password you entered is incorrect. Try entering your information again."

Anything else I can try?