Just a question: Do you think the requested URL referring to one of your own Tiny MCE-plugins or does it seem to be requesting something built in in EPiServer? Seems like it's missing the plug-in name in the path where the double slash is. We have special handling for third party add-ons since the upgraded Tiny MCE version now is located inside the CMS add-on and therefore we need to handle third party add-ons inside the util folder. But there might of course be a bug here.

I don't have any TinyMCE plugins at all. So I think it's trying to request something in EPiServer, but suddenly missing the plugin-name for some reason.

Looks like you have a TinyMCEPlugin class without PlugInName set. Could you please dump and post your tinymce settings?

Exactly what settings are you intresting in? Here are the settings in EPiServer anyway:

https://docs.google.com/file/d/0B1GzO91ZdUbzcWJCamlIR1Nja0k/edit?usp=sharing

I saw an enabled plugin that doesn't have translated name, very likely a tinymce plugin from your site. Could you please search for all TinyMCEPlugin or TinyMCENonVisualPlugin in your code base?

You are right! I DID have a plugin that I forgot. I'm using a code foundation for all EPiServer sites which includes a small plugin just to enable iframes. So, all I needed was to set a pluginname and add that folder to the plugin-folder.

Yes, as far as I know, tinymce is not fault tollerent on loading plugins on FF and Chrome. IE surprisingly works.

Just found another folder missing; "/util/Editor/tinymce/plugins/epicontentfragment/editor_plugin.js", and that's a plugin that I'm sure I haven't created myself. This i requested only when editing dynamic properties and gives me a 404. I solved it the same way, created the folder and put an empty editor_plugin.js-file in it.

I got this error as well. How did you resolve it?

There are a couple ways you can resolve it. You could:

1) Create an empty editor_plugin.js file in the location that is being requested to prevent the 404

2) Create an empty file handler and set up configuration so the handler only targets the requested path. Check near the end of: http://cjsharp.com/blog/2013/04/15/how-to-add-valid-elements-like-iframes-to-tinymce-in-episerver-7/

3) As Joshua Folkerts commented in the above link: "You can also add a virtual path to the episerver.config file to handle the new url and map it another location without using a handler. This way, you add the vp and reference the same empty js file for all plugins."