For some reason, a user who has full access to all mailboxes is unable to expand other users boxes in Outlook view. Also his mailbox doesn't always open. When it is not in cached mode an error is displayed:
In cached mode the mailbox opens but usually complains that it is disconnected. Connecting to Exchange fails until it suddenly succeeds.
The problem is very similar as in this topic: https://www.experts-exchange.com/questions/29125111/Intermittently-cannot-open-Outlook-The-set-of-folders-cannot-be-opened-Network-problems-are-preventing-connection-to-Microsoft-Exchange.html
However in my case the mailbox migration to a different database didn't help.
Our on premises Exchange environment was migrated from Exchange 2010 to 2016 and after that these problems have followed users occasionally. Setting a registry value to prefer IPV4 over IPV6 brought some stability but even after that we've had some reports of these problems. I have a feeling that the problem might be somehow MAPI related and it has to do with the amount of mailbox objects etc.. I know there is a registry value that is mentioned in some discussions that can be raised but others are against messing with Exchanges registry.
Now the full access user and it's mailbox is used for a software that needs to sync data from other users mailboxes. When the sync process tries to run an error is displayed:
Software User: username
PIM Account: username
Error Message: A MAPI related error occurred (MAPI_E_NETWORK_ERROR). Component = Microsoft Exchange Information Store, Description = Network problems are preventing connection to Microsoft Exchange.
Error Location: IDC_MAPISession::LogonInte
Also this topic has similar problem: https://community.spiceworks.com/topic/2162356-outlook-exchange-randomly-disconnects-users
Any suggestions on what to check?
Edit. We only have one Exchange server that holds all roles. Exchange 2016 with CU 15. All clients are Win10 x64 with Office 2016 x64