I figured out what is causing it: in the new 3.9 WP, because I use the ro_RO localization, it somehow forces my
cforms plugin to have the same language file in it's folder
domain.tld/wp-content/plugins/cforms/js/langs/ro_RO.js, which I do not have by default.
"Failed to load: http://domain.tld/wp-content/plugins/cforms/js/langs/ro_RO.js" tinymce.js:7761
As I could see, the plugin uses simple names
en.js / de.js / fr.js. I manually created the
ro.js and it still does not make tinymce work; renaming it to
ro_RO.js gets the things working (but it might not make the plugin work if it needs it's own filename standards).
I do not get it why it forces me to create the language file for a plugin. It should first check if it exists and only then should use that file, just like localizaton works with .mo files, the website does not fail if you miss the file, it will only not use any translation :)