When running Client Profile Updating Utility (CPUU) against a Outlook 2010 64 bit (x64) client the following error is found in the CPUU log
10/4/2017 9:16:21 AM 9268 [Info] Default mail client: Outlook64Bridge.
10/4/2017 9:16:21 AM 9268 [Info] Temporarily changing the default mail client to 'Microsoft Outlook'.
10/4/2017 9:16:21 AM 9268 [Error] Cannot set default mail client.
Standard result: 0x00000005, Description: Access is denied.
Outlook 2010 x64 version has a known issue where is has issues being designated as the Default Mail Client. This is especially problematic when a 32 bit application tries to leverage the Default Mail Client role to perform tasks as with CPUU trying to set this role.
Updated versions of the Outlook 2010 x64 version should be attempted first with the latest fixes.
There are several workarounds for the issue as outlined in the forum link below including adding registry keys and raising the users access level on the machine by adding them to various groups: