Kai,
I tried this morning and the key seems to be working correctly. Granting the user read access or full control, either way the dtrace and the result is the same.
7758 08:39:18.749 [26668] (AdminService) <6464> EV:L CPermissionsHelper::IsFolderSecure Entry. Checking folder 'C:\Users\VAULTA~1\AppData\Local\Temp\'
7759 08:39:18.750 [26668] (AdminService) <6464> EV:L CPermissionsHelper::LoadExceptionsFromRegistry Entry
7760 08:39:18.750 [26668] (AdminService) <6464> EV:L CPermissionsHelper::LoadExceptionsFromRegistry: Found temp folder exceptions reg key.
7761 08:39:18.750 [26668] (AdminService) <6464> EV:L CPermissionsHelper::LoadExceptionsFromRegistry Exit
7762 08:39:18.750 [26668] (AdminService) <6464> EV:L CPermissionsHelper::IsSidAllowed Entry, sid type : 1
7763 08:39:18.750 [26668] (AdminService) <6464> EV:L CPermissionsHelper::IsAllowedAdminSid Entry
7764 08:39:18.750 [26668] (AdminService) <6464> EV:L CPermissionsHelper::IsAllowedAdminSid Exit :passed sid is not a well known admin sid.
7765 08:39:18.750 [26668] (AdminService) <6464> EV:L CPermissionsHelper::IsSidAllowed Testing user type sid...
7766 08:39:18.750 [26668] (AdminService) <6464> EV:L CPermissionsHelper::IsSIDInGroupToken Entry
7767 08:39:18.750 [26668] (AdminService) <6464> EV:L CPermissionsHelper::IsSIDInGroupToken Exit.
7768 08:39:18.750 [26668] (AdminService) <6464> EV:L CPermissionsHelper::IsSidAllowed Exit. Username: user3
7769 08:39:18.750 [26668] (AdminService) <6464> EV:L CPermissionsHelper::IsFolderSecure SID is not in the approved list, proceeding to registry lookup..
7770 08:39:18.750 [26668] (AdminService) <6464> EV:L CPermissionsHelper::IsSidInRegistryList Entry
7771 08:39:18.750 [26668] (AdminService) <6464> EV:L CPermissionsHelper::IsSidInRegistryList: passed sid found in registry exception list.
Please gather a dtrace of AdminService while reproducing the issue and post it here, so we can see what's different about your situation. Please also include exact string from your TempFolderExclusions registry value.