Shared hosting, multiple sites, can't log in to WP due to .htaccess redirection

Question

I've got a shared hosting account with 3 separate domains on it. Each site is in its own directory, so via .htaccess there is a rewrite for each domain name to the corresponding folder.

When I try to log in to WP, the redirect URL that shows up in the address bar has the domain's folder in it.

Ex: www.domain1.com is under folder /domain1

Login redirect after login is http://www.domain1.com/wp-login.php?redirect_to=http%3A%2F%2Fwww.domain1.com%2Fdomain1%2Fwp-admin%2F&reauth=1

So it just brings me back to the login screen with no error message or anything...if I reset the password, however, I can log in successfully. But upon logout I get in the redirect loop again.

EDIT - this code is in .htaccess located in /public_html/. There is also another .htaccess inside /public_html/domain1/ with the standard Wordpress lines.

RewriteEngine on

RewriteCond %{HTTP_HOST} ^(.*)domain1.com$ [NC]
RewriteRule ^(.*)$ /domain1/$1 [L]

RewriteCond %{HTTP_HOST} ^(.*)domain2.com$ [NC]
RewriteRule ^(.*)$ /domain2/$1 [L]

RewriteCond %{HTTP_HOST} ^(.*)domain3.com$ [NC]
RewriteRule ^(.*)$ /domain3/$1 [L]

Answer 1

@define('ADMIN_COOKIE_PATH', '/');

Putting this at the top of wp-config.php allows me to log in. Not sure if this is the proper way to set everything up, but it works...

EDIT

That only solved half of the problem. Permalinks were then broken.

@toscho had the proper suggestion for anyone who may see this and has a similar issue.

However, this hosting provider doesn't allow virtual hosts, so if you're in a similar situation what ended up working for me was a few more .htaccess lines (see below) combined with the wp-config modification above.

# .htaccess main domain to subdirectory redirect
# Copy and paste the following code into the .htaccess file
# in the public_html folder of your hosting account
# make the changes to the file according to the instructions.
# Do not change this line.

RewriteEngine on

# Change example.com to be your main domain.

RewriteCond %{HTTP_HOST} ^(www.)?example.com$

# Change 'subdirectory' to be the directory you will use for your main domain.

RewriteCond %{REQUEST_URI} !^/subdirectory/

# Don't change these line.

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d

# Change 'subdirectory' to be the directory you will use for your main domain.

RewriteRule ^(.*)$ /subdirectory/$1

# Change example.com to be your main domain again.
# Change 'subdirectory' to be the directory you will use for your main domain
# followed by / then the main file for your site, index.php, index.html, etc.

RewriteCond %{HTTP_HOST} ^(www.)?example.com$
RewriteRule ^(/)?$ subdirectory/index.php [L]