We discovered a new bug in App Volumes:
When you move the appstack templates to a new datastore and afterwards rename/delete the LUN where the templatee originally were located, then it is impossible to update an appstack which was originally created on the initial LUN.
The workaround is to change the reference of the original LUN in the SQL database for each appstack which was created while the app.
The version we are using is 2.9