Thanks for this link http://www.sitefinity.com/documentation/documentationarticles/designer-s-guide/page-templates/creating-a-template-using-master-pages
It seems the problem was cause by the RadScriptManager I had in my .master
I also had a ContentPlaceholder in the <head> tag (VS default when you create a .master) but removing this did not do the trick.
So if anyone has the same problem
a) remove the ContentPlaceholder in the <head> section
b1) remove the RadScriptManager or
b2) replace the RadScriptManager with the regular ScriptManager