Currently working on this and can confirm that changing %APPDATA%\Shotgun\desktop\install\app_store\tk-framework-desktopstartup\v2.1.13\python\shotgun_desktop\location.py
line 71 return yaml.load(location_file, Loader=yaml.FullLoader) or dev_descriptor
to the following
return yaml.load(location_file, Loader=yaml.FullLoader) or dev_descriptor
return yaml.load(location_file) or dev_descriptor
fixes the error on SG Desktop launch. Obviously directly modifying the python files inside tk-framework-desktopstartup is not advised but does seem to point to the distributed yaml inside %APPDATA%\Shotgun\desktop\install\app_store\tk-framework-desktopstartup\v2.1.13\python\tk-core\python\tank_vendor not being a supported version.