It will be trimmed to https://www.youtube.com/c
Then if you do refresh, bookmark it or reopen last closed tab, it gives an error.
The same is for URLs of type user: https://www.youtube.com/user/professorleonard57
This is in both Opera and Firefox.
Pretty dirty trick. I've noticed that a lot of the newer channels tended to be put behind long URLs of random characters as well, so they aren't easy to remember.