WA_getDocumentDOM is not defined
I just found the solution accidentally.
Webassist extensions installed with Administrator rights. So only Admnistrators have this C:\Program Files (x86)\Adobe\Adobe Dreamweaver CS6\configuration\Shared\Spry\DesignTime\EditingUtils.js full access. A restricted user (as a normal daily windows user) has only Read & Execute and Read access. Give your restricted user full access to this file and it will work.
My config is Windows 7 Prof, with CS6 Design Premium.