Alright, this is kind of a strange problem.

I have two sites - bestcatholicposters.com and bestscientists.com, both built with the latest version of Zencart 1.39h.

Since bestscientists is the least trafficked, I installed ceon 4.0.1 on it.
Worked like a charm.
LOVED it.

So, I went to bestcatholicposters.com and installed it there.
Everything exactly the same, as far as I can tell.

Whenever I cut-and-paste the code into the .htaccess, the site responds with 500 Internal Server Error.

## BEGIN CEON URI MAPPING REWRITE RULE

RewriteEngine On

# ONLY rewrite URIs beginning with /zencart/
RewriteCond %{REQUEST_URI} ^/zencart/ [NC]
# Don't rewrite any URIs ending with a file extension (ending with .[xxxxx])
RewriteCond %{REQUEST_URI} !\.[a-z]{2,5}$ [NC]
# Don't rewrite any URIs for some, popular specific file format extensions,
# which are not covered by main file extension condition above
RewriteCond %{REQUEST_URI} !\.(mp3|mp4|h264)$ [NC]
# Don't rewrite any URIs for some specific file format extensions,
# which are not covered by main file extension condition above
# Uncomment the following line to apply this condition! (Remove the # at the start of the next line)
#RewriteCond %{REQUEST_URI} !\.(3gp|3g2|h261|h263|mj2|mjp2|mp4v|mpg4|m1v|m2v|m4u|f4v|m4v|3dml)$ [NC]
# Don't rewrite cPanel directories
RewriteCond %{REQUEST_URI} !/cpanel [NC]
RewriteCond %{REQUEST_URI} !/frontend [NC]

# Don't rewrite admin directory
RewriteCond %{REQUEST_URI} !^/zencart/zcadmin [NC]
# Don't rewrite editors directory
RewriteCond %{REQUEST_URI} !^/zencart/editors/ [NC]
# Don't rewrite image files directory
RewriteCond %{REQUEST_URI} !^/zencart/image files/ [NC]
# Handle all other URIs using Zen Cart (its index.php)
RewriteRule .* zencart/index.php [QSA,L]
#RewriteRule .* index.php?%{QUERY_STRING} [L]

## END CEON URI MAPPING REWRITE RULE

I've eyeballed comparisoned everything I can think of - configuration and configuration_group, both .htaccess files, but I can't see why one works and the other doesn't.
I may have screwed up the second installation, but since it worked on the first one, I gotta believe it was something subtle.

Thoughts?