We are having a strange problem using calendar invites where if the client is using cached Exchange mode and they open a calendar invite sent from another user on the same Exchange server they can open the calendar invite, but if the client is not using cached Exchange mode and they try to open the calendar invite they get the message "Outlook cannot open item: Subject".
We have already tried deleting temporary files and that did not fix the problem.
We sent a calendar invite from another domain to someone in the domain with this issue and they were able to open the calendar invite with a client using cached Exchange mode and with a client not using cached Exchange mode.
We will take a closer look at the Exchange server to see if "Rooms" or other Resource Mailboxes are involved but wanted to see if anyone had some different ideas.
The Exchange server is Exchange Server 2010 (fully patched) and multiple employees have this problem with Outlook 2013 and Outlook 2016.
We do not believe it is related to Resource Mailboxes because not every calendar invite that has this problem is related to a Resource Mailbox.