I recently ran in to a problem running the DirSync tool with Office 365.  My account was an Enterprise Administrator, as well as a member of the MIISAdmins local group.  The error I got after attempting synchronization was “Set-CoexistenceConfiguration was unable to modify the source properties.  See the event logs for more detailed information.”:

Upon opening the Event Viewer, as the ever-so-meaningful error instructed me to do, I found the following Errors logged:

• A Constraint violation occured. (Exception from HRESULT 0x8007202F)
• Set-CoexistenceConfiguration was unable to modify the source properties.

I determined where the error was occurring by running the AD Insight tool, which is part of the Windows Sysinternal applications and can be found here: http://technet.microsoft.com/en-us/sysinternals/bb897539.aspx.

The error indicated that I was unable to modify an attribute within the domain, which lead me back to checking permissions.  I found that though my system account was part of the Enterprise Administrators in our domain, Enterprise Admins were not included in the built-in Administrators group for our domain. Once I added the account to the Administrators account and retried the directory synchronization, everything worked perfectly.

Then click retry and it works perfect !

ENJOY.

And if you encounter any problems feel free to comment / contact me.