Unusual login problem when adding new plugins

Thanks in advance for any advice.

I have no problem logging in to my site, with my normal password, but when I try to add a new plugin via the admin panel, it sends me to another wordpress login page, for which my password doesn’t work. I’ve tried changing the password, logging out of everything and restarting, and still cant figure it out. Any advice?

heres the site, if it helps.