I recently ran into an issue on a VFS under GIT source control where I had checked out a different branch and was then unable to get to the Opportunity quickforms via the Application Architect. I was receiving an error like:
Exception has been thrown by the target of an invocation. (mscorlib)
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There is an error in XML document (2, 2). (System.Xml)
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<modelItemIndex xmlns=''> was not expected. (ssjhvcaf)
The issue was that another file called the modelindex.xml references by ID the code properties in quickforms. Apparently this XML file was not properly contained in my source control branch. Deleting this file and then re-opening the Application Architect caused this file to be re-created and I was then able to access the quickforms.
This file can be found in the root of the model directory in your VFS (VFS\Model\modelindex.xml).