I have this same problem on 1and1. An earlier commenter listed the content of his htaccess. These particular lines are important:
——–
order deny,allow
Allow from XXX.XX.XX.XXX
deny from all
——–
I checked my own htaccess file and it included these lines except for the “Allow from” line.
So, I googled “my IP address” to get my public/external IP address for my home. Then I added the “Allow from” line followed by my IP address. Saved. Uploaded.
It took a few attempts/refreshes to clear whatever cache, but then I was – surprise surprise – ALLOWED to log in. At least when connected to the network at my home.
Hope this is helpful.