First I should say that there are many questions and answers on this topic around the web but I'm not finding anything close to what's happening here so I really hope someone here is able to help.
I have the wysiwyg disabled by default and when I click on Insert Image the pop up comes up as expected but is not fully rendered, see attached screen shot.
The console is saying Uncaught SyntaxError: Unexpected identifier, see attached screen shot.
I have disabled all extensions to rule out conflict + both media and wysiwyg folders have 777 permissions.
EDIT: I have tested Chrome, Firefox and Safari on Mac + Chrome on PC, all with the same result.
Just let me know if I left out any necessary info!
Big thanks in advance!
Best Answer
So after a day of diving into this issue; disabling modules, enabling modules, removing changes in local etc etc, I found the problem.
I used an extension called PhPro_Translate to do a large amount of translations for front and backend from English to Swedish and so in my std install I have just inserted all the rows of translations into core_translate (as the extension does). It turns out that one of the 80 or so translated strings under the Mage_Cms module cause the error. I honestly did not have the energy to look into exactly which string as I still have 3400 rows working :), but I have a vague idea it could have been a Swedish word with umlauts.
In the very unlikely event that someone else runs into the same problem, that's the answer!
Cheerio
UPDATE: Since this could have implications in other contexts, here's what caused the js error. In one of the translations I had used single quotes around a word to denote it was the original English term but I had forgotten to escape the quotes. Voilà!