Any reauthenication sends me to infinite login loop

Hi there.

I have a WP blog on mydomain.com. I also have add-on domains on the same account, and to keep things clean in the folder structure I set up the blog in a subdirectory of its own.

So my blog is in mydomain.com/mydomain.

However I wanted to be able to access it using mydomain.com so I set up the htaccess as follows. This works great and I can browse to mydomain.com and see my blog which is actually in a subdirectory

Options +FollowSymLinks
RewriteEngine on
RewriteCond %{HTTP_HOST} ^(www.)?mydomain.com$
RewriteCond %{REQUEST_URI} !^/mydomain/
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /mydomain/$1
RewriteCond %{HTTP_HOST} ^(www.)?mydomain.com$
RewriteRule ^(/)?$ mydomain/ [L]

However there is an annoying issue with a looping login.

If I go to https://mydomain.com/wp-admin I then get directed to login at:
https://mydomain.com/wp-login.php?redirect_to=http%3A%2F%mydomain.com%2Fmydomain%2Fwp-admin%2F&reauth=1

Which when I login I get directed back to the same login page. Note the double-up of mydomain.com/mydomain in that link.

This hasn’t been to much of a problem as I could just go to /wp-login.php and login from there.

But now I am trying to use Configure SMTP plugin, and it also seems to be trying to get me to authenticate myself. And I get the same problem – I get sent out to the login page, and redirected back to the login page again.

Is there something in my htaccess I can change so I don’t have this login loop problem. I’m new to htaccess so this is all a bit cryptic to me

Thanks a lot!