403 Error when trying to log in

Hey guys.

So, my girlfriends grandfather has a hobby-website, which he would like me to help him with.

But when I try to access the site via /wp-admin I get a 403 message.

Still after he set me up as administrator of the website and I got an email with an activation-link, I still get the 403.

This is what it says:

“The following error occurred:

You are not permitted to access the requested URL.
Please contact the Webmaster with any queries.”

Where does the problem lie?

Best regards