There are 2 steps. First you click on login on the Vivaldi site, then a new site appears and there you input your username and password. Then you click login on this page and -- nothing happens for a while, because Vivaldi is processing this request. If you get inpatient and click login again, you will get the authentication error you mentioned. Then only going back to a normal vivaldi site, reloading, and doing the whole thing again will let you login. So again, after hitting login be patient and wait for the site to do its thing, then it should work out fine without the error.
About the pasting of passwords I can only say that i could paste fine in this field, but I actually let Vivaldi save the password for vivaldi.net, therefore the login information is already filled out in my case.
iMac El Capitan user here just experienced the same thing. Thanks for the tip. I'll git used to it.