Hi there!
UltiDev Cassini runs as windows service under Local System account. Most folders on a computer are accessible to Local System account. Please check whether Local System already has access to the folder with your application, and if you get the 403 when trying to reach Cassini Explorer, check if Local System has access to the "C:\Program Files\UltiDev\Cassini Web Server Explorer" folder. If Local System already has access to those folders, please post the content of the "C:\Documents and Settings\All Users\Application Data\UltiDev\Cassini\CassiniMetabase.xml" file - it may provide clues as to why things are not working.
Best regards,
UltiDev Team.
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