Upload works, but can’t edit gallery afterwards

I’m having an issue with posts with gallery short code in them. After creating a post and uploading files using the media uploader from my computer, I get a “Not found” error on file /cp_wp/wp-admin//media-upload.php. Note the double slashes in front of media-upload.php — I can’t find where the extra slash is coming from. Other than that, the path is correct.

My .htaccess file is located in wp-admin and contains:

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

My install is located at mydomain.com/cp_wp, so the rewrite rules above look to be correct as far as I can tell. My workflow for creating a post and uploading images for a gallery:

New post
Add title and text to the post
Click insert media-images
Select Files (flash uploader)
Select 5 images, wait for upload/crunching
Click “save all changes”
In the Gallery section below, click Link Thumbnails to Attachment
Order by Name
Click Insert Gallery

Thus far, everything works. The post displays when I visit the site, the gallery works (which a could of minor behaviors that differ from dev that I can’t figure out, but overall it works), etc. However, if I return to the admin site and edit the post, when I click on the gallery section and click “Edit Gallery”, I get the error:

Not Found
The requested URL /cp_wp/wp-admin//media-upload.php was not found on this server.
Apache/1.3.41 Server at http://www.mydomain.com Port 80

I’m at a total loss because I did not encounter this issue at all on my local development machine during testing, and I have replicated my files and settings exactly. It only occurs out at my ISP.

Any help would be greatly appreciated. Many thanks and best regards,
Terry