Solved it. A reference to jquery used in the sites code on the startpage caused the error. using jquery together with composer is a little jumpy. 

Now I get another script error but only in IE. The error occurs when clicking edit on a composer element.

Type mismatch i Dropit/plugin/extension/ui/x3.min.js.The code that generates the error is

contentFunctionNode.setAttribute("typename",this.TypeName);

Composer works fine in Firefox and Chrome but on Chrome there is a flash of a page can not be found (dropit/plugin/extension/ui/blank.htm) before the editpage is loaded.

More information about the problem. I was a bit confused that when loading Composer the composer functions was not wrapped with the correct colors for Content, Global and Layoutblocks.

Everything is white and checking the code with FireBug I see that the Containers don't get the correct Css class set.

The classnames becomes

TitleContainernull, TitleContainerBlocknull, HeadAreaLeftnull, HeadAreaRightnull. So the last value is not correct.

Perhaps this behaviour can help someone help me figure the issues we're having

Hi, Tobias!

I also upgrade my site from CMS 5 and Composer 3.2.5 to CMS 6 and Composer 4 and got this errors. I've just solved them out! (Mind that I did not develope my site from the scratch, got it initial from swedish Dropit)

The problem is that You probably use in solution some class (in my case it is Foundation.Extension.FunctionBase nested Dropit.Extension.Core.BaseContentFunction) that overrides!!!! renderControl method using the knowledge of realization of the Dropit.Extension.Core.BaseContentFunction.

BUT!! in 4 version this realization dramatically (in Your and my case especially dramatical) changed!!!

instead of "ft" attribute become "typename" and added attribyte "remotesite".

so, found method AddAttributesForExtension in 

Foundation.Extension.FunctionBase

 and replace to the next

protected virtual void AddAttributesForExtension()

{

if (IsInEditMode() && ContentFunctionData != null)

{

Attributes.Add("pid", ContentFunctionData.FunctionID.ToString());

Attributes.Add("wid", ContentFunctionData.WorkID.ToString());

Attributes.Add("typeid", ContentFunctionData.FunctionTypeID.ToString());

Attributes.Add("guid", ContentFunctionData.Guid.ToString());

Attributes.Add("isglobal", (ContentFunctionData.IsGlobal ? "1" : "0"));

Attributes.Add("typename", ContentFunctionData.FunctionType.ToString());

Attributes.Add("remotesite", ContentFunctionData.RemoteSite);

Attributes.Add("class", "Ext3ContentFunction");

}

}

thanks for the input, solved the issue. The site we used also came from Dropit.

You solved it earlier than I posted my solution?

A colleague took over this project and when i saw your response I looked up if the issue was solved and he confirmed the solution was like yours. Havent had time to followup the issue since I've been involved in other projects.