This issue has caused me so much pain over such a long time, and I have finally found the cause. Nothing to do with Colbox (or CF for that matter).
It turned out to be an issue with the way I had set the site up in Apache.
The site was in a folder called “/Users/jason/Sites/Groups/site”.
In my apache httpd-vhosts.conf, for this sites config i had DocumentRoot “/Users/jason/Sites/groups/site”.
The folder “Groups” had an uppercase G, and the document root reference in vhosts had a lowercase g. When I changed the folder to ‘groups’. the problem disappeared…, have confirmed this fix on other sites as well, so 99% sure this was causing me the problems.
Thanks for the help provided!!
Jason