Situation: Exchange 2016 cu18 with all the available patches.
Outlook: 2016 16.0.4966.1000 32-bit version.
No PST files. No cached mode. On-premise Exchange. Exchange is online all the time.
There is already a lot of information about this kind of issue but none does the trick for us.
What we see: One outlook profile. User has his own mailbox. And also in his profile he has another mailbox we use the credentials of that mailbox.
Sometimes when he opens his Outlook he gets the popup that Outlook cannot be opened and that the set of folders cannot be opened.
We tried changing add-ins, safe mode, new profiles, ... but the the important thing is that if we do nothing it solves itself after minutes, hours, .... although at that moment there are no issues with the connection of the Exchange server because the other 500 people can still work.
What we likely are seeing is that while opening Outlook the credentials are asked from the mailbox which is connected to the account of the user. So to be clear: I logon to windows with accountAA, I open Outlook and the primary mailbox is of course the mailbox of accountAA, but credentiasl are asked via a popup. In that popup I see the credentials of that added secondary mailbox. Of course I can change that to accountAA, fill out the password an Outlook opens.
Sometimes there is no popup at all and Outlook does not open.
When creating a new profile sometimes it opens without a problem, sometimes it doesn't. And when I go back to the original outlook profile it goes open without a notification.
Extra: there are only a few people who has seen this popup but I cannot say if it is always the same group of people. Some people reacts more often then others, that is true.
So I want to focus on the extra mailbox. Could this be an issue? We have a lot of people who have extra mailboxes. Yes some more than 10 in one Outlook profile. But as said earlier not everybody with extra mailboxes does have issues.
We also removed some Windows profiles and created them from scratch. but does not solved it completely.
What I want to ask is: could it be an issue on the Exchange server. related ot the virtual directories? Maybe our mapi settings are not as wished?
Now the authentication is as follows: windows authentication-ntlm and windows auth-negotiate are on. At IIS level default web site / mapi / only windows auth is on. At back end mapi has anonymous auth and forms auth on. Is that correct? Can't remember I have changed that in the past.
Hopefully we get a tip, even how small, and we can find the solution for this strange behaviour.
Best regards
Kurt

