I have a network with 3 client machines using outlook 2010 and sharing a calendar. Its been working fine for months. As of this morning it no longer syncs.. 

I have tried all the "Turning off the caching of shared folders in Outlook on Windows computers prevents this issue."

Hi,

Have you checked the shared calendar from OWA? Does it sync over there?

Have you tried remove the shared calendar and then reopen it again to see the result?

Please check if the permission of the calendar is correct. We can also try removing the permissions and then reassigning them again to see the result.

In addition, do you have any third-party add-in installed recently which may interrupt the synchronization? If so, please go to File > Options > Add-ins, disable suspicious add-ins and then test it again.

Regards,

Steve Fan
TechNet Community Support