Scratched my head over this one for quite some time.
Using a vanilla installation of Joomla 3.4.8 with Gantry framework 5.2.11 using Hydrogen theme 5.2.11. I was trying to add a “Single Article” to a menu link, but every time I would click the “Select” button, a lightbox displaying the site template with a 404 Not Found error would be displayed.
Server logs confirmed the 404:
xx.xx.xx.xx – – [27/Feb/2016:00:00:23 -0500] “GET /administrator/index.php?option=com_content&view=articles&layout=modal&tmpl=component&function=jSelectArticle_jform_request_id&7792<…snipped…>0fb=1 HTTP/1.1” 404 6810
Troubleshooting suggested that the “jSelectArticle_jform_request_id” portion of the URL being invoked by the “Select” button was related to whatever was causing the problem.
An article I found suggested it may have had something to do with the use of SEO friendly URLs, so I tried disabling the Joomla “Search Engine Friendly URLs”. That resolved the 404 error appearing, but the lightbox that was displayed was now just blank. Still no joy.
That led me to think the URL Rewriting feature may have something to do with it. I *enabled* the “Use URL Rewriting” option in Joomla Site Settings and… voila! Problem solved.
It seems the “Use URL Rewriting” SEO setting must be enabled in order for the article selection feature to work properly.
At least, that’s what the solution was in my case. Your mileage may vary.