Do you have the MODX TinyMCE extra installed (or incompletely uninstalled)?
Have you modified the npElFinderInit TPL chunk? A typo there might cause this.
These lines in that chunk are suggestive:
// file is the URL of of the npElFinder resource
If the HTML extension is wrong it will crap things up. The code queries the modx_content_types table in the DB to get the np_html_extension. Could you look in that table and see if HTML is the first entry and if its value field matches the extension used in the sites page's? Also, if that resource is a container and containers don't have an extension, that could be the problem.
Another thought: If it's a multi-language site, maybe the site_url tag should be uncached:
I've never tested with multiple contexts, so that by itself could cause an issue, though if it was working before, that's probably not it.