Office365 Password-Prompt once a day

Question

We have installed Office365 (with hosted Exchange and AD-Sync) on a client's network. Some of them experience a weird behaviour of outlook. The user arrives at this desk, boots up his computer and starts outlook. Then he gets prompted for a password. He puts it in and it works. The next day it happens again.

We tried closing outlook and restarting it -> didn't ask for password.

We tried rebooting the computer -> didn't ask for password.

Clearing out the Credential Manager or disabling cached mode didn't help. Any ideas what the source of this problem could be? My google-fu only found one other topic with this kind of behaviour (once a day prompts) and that was on ESET forum with no solution. We use ESET too, but the problem exists before we switched the client from Kaspersky to ESET.

I cannot change his authentification type in his settings. The option doesn't exist in his outlook. I thought it might use Kerberos and after 10 hours (max duration of a ticket) it would prompt again for a password.

Clients uses Windows 10 64-bit with Outlook 2016.

Answer 1

We tried a different set of approaches and it seems like I was a bit too hasty with the Exchange-Cache-Mode. When we first disabled it, then he got a different set of error-notifications, he said. I didn't go into details with him, when I should have.

The problem was fixed by disabling the Exchange-Cache-Mode in Outlook. We waited a few days and asked the customer again. He said no further password-prompts happened. Then enabled the Exchange-Cache-Mode in Outlook again and waited another couple of days. Still no password-prompts. I'm not sure whether the waiting-time between disabling and re-enabling is relevant here, but that's at least how I did it. I'm still a little confused on why this solved the problem, but right now I'm just happy that it did.