It's quite weird, because this issue, I mean your account recognize but not your password, is exactly the issue with the old wordpress package.
Oh, but maybe there also a issue with the apps lists (because we had some problems with one of our server)
So, can you try to upgrade your apps lists before, then recheck your upgrade please.
Or, if you can, try to upgrade by command line.
sudo yunohost app upgrade wordpress -u https://github.com/YunoHost-Apps/wordpress_ynh
Ok, I should read what I have previously written before...
I guess you made the upgrade from the testing branch, like I've proposed before ?