Outlook 2016 Could not log on

I have 3 computers using the same Office 365 email account, and all three computeres experience the same problem.

Just about every day Outlook does not start and the error is that "Could not log on. Check that you are Connected to the network, and using the correct server....... (freely translated).
I can make a new profile, and it will work for a day or so before the same problem comes back.

Tried using the Office 365 Assistant to correct the problem, and it does the job well, but the problem still comes back.
The Assistant tells me that there is a problem with "Autosearch", and that it will update the local Autoseach file with the newest settings from the server. It successfully does so, and Outlook work just fine until the local autoseach file get corrupted again.

I've been trying to figure out where this local autoseach file is located on my disk so I can analyse what happens to it, but have been unable to do so.
Does anyone know what file and location I am looking for?

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Tom CieslikIT EngineerCommented:
Are you sure that on affected computers there is no external "like google" DNS set on NIC and there is not VPN connection set and problems you've is not related to some other computer connections like separate WiFi card from different VLAN ?
Vasil Michev (MVP)Commented:
The O365 assistant probably refers to AutoDiscover. You can do an internal test via Outlook's "test e-mail autoconfiguration" wizard (hold the CTRL key, right-click on Outlook's icon in the tray, select test e-mail autoconfiguration, enter UPN and password, clear the two Guessmart checkboxes and run the test), then compare the results with the external test you can run via ExRCA (https://testconnectivity.microsoft.com/ -> O365 tab -> Autodiscover test).

How big is the mailbox/Outlook OST file?
This is probably an Autodiscover issue. Make sure that there are no other "autodiscover" DNS entries in your hosting provider, sometimes they appear as SRV record. If you found any other autodiscover aside from autodiscover.outlook.com try to remove them but keep a backup copy.

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This is not exactly what you are experiencing but during out POC of o365 and Outlook 2016. We found that Outlook 2016 was asking using to login after opening Outlook. We corrected this by enabling modern authentication in o365. Like I said not exactly was you are experiencing but food for thought
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