Wordpress 403/404 Errors: You don't have permission to access /wp-admin/themes.php on this server

A few years late, but I have a solution for the most recent version of WordPress, which has this same issue (WordPress-generated .htaccess files break sites, reuslting in 403 Forbidden error messages). Here's that it looks like when WordPress creates it:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress

The problem is that the conditional doesn't work. It doesn't work because the module it's looking for isn't .c, it's .so. I think this is a platform-specific, or configuration-specific issue, where Mac OS and Lunix Apache installations are set up for .so AKA 'shared-object' modules. Looking for a .c module shouldn't break the conditional, I think that's a bug, but it's the issue.

Simply change the mod_rewrite.c to mod_rewrite.so and you're all set to go!

# BEGIN WordPress
<IfModule mod_rewrite.so>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress

We found the asnwer of at techieshelp..its the htaccess thats the issue...

http://www.techieshelp.com/you-don%E2%80%99t-have-permission-to-access-wp-adminpost-php-on-this-server/

Just to follow up, problem solved! I mentioned mod_sec settings for my server as being the possible culprit as suggested and they were able to fix this issue. Here's what the tech agent said to tell them when you go to support:

Just let them know you need the rule 340163 whitelisted for domain.com as its hitting a mod_sec rule.

Apparently you will need to do this for each domain that is having the issue, but it works. Thanks for all the suggestions everyone!