When editing a page and clicking the composer tab, the page loads very slowly. When it finally does, the Composer Toolbar is empty and unclickable. Clicking on the tabs generates the following in the console: 

TypeError: ExtensionEditTools_ctl00_oFunctionBar_oMultiPage.SetPageIndex is not a function

I've tried to:

• Update composer to 4.1
• Remove css and javascripts

... but nothing has helped. Does anybody has a clue as to why? I'm running EPiServer 6.1, Composer 4.1