outlook 2016 asks for credentials

We are using exchange with outlook 2016 and been asked for credentials since yesterday.
It seems that the used mailadress in outlook is also registered at office.com for some reason.
After entering the password from office.com the login-mask from outlook won't return but outlook still says that it is not connected to exchange. After clicking the information at the bottom it changes immediatly to "online with exchange".

We also updated our office to the newest version, but the error ist still there.
Restarting doesn't solved the problem, either. And therefore the starting process last at least 5 minutes.

It seems that outlook looks somewhere for my account but can't find any.
Can any proof this behaviour or solve?

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Pete LongTechnical ConsultantCommented:
Are all the machines on a corporate domain? You need to ensure your 'autodiscover' is pointing to your "On-Prem" Exchange, and not the O365 instance.

For internal clients if you have a windows domain simply setup an SRV record Exchange AutoDiscover Errors – Creating an AutoDiscover SRV Record

loosainAuthor Commented:
autodiscover is working and pointing to on premise server. Only some workstations are asking for the credentials. Not all clients have the Problem, so dns and server settings shouldn´t be the problem... It must have been something with the client.
David HaycoxConsultant EngineerCommented:
We are seeing this also.  We believe it's due to a Microsoft update.  It only affects Outlook 2016.  For some reason the Outlook client is trying to authenticate against "outlook.office365.com" (which you'll see if you look in the cached credentials list on the client) when the email account is hosted on an on-premises Exchange server (Exchange 2013 or 2016).

The only solution we have found so far is to uninstall Office 2016, then reinstall it and decline any updates.
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David HaycoxConsultant EngineerCommented:
There's a possible solution listed partway down this thread:



The fix is to create the following Registry keys on affected machine:

Navigate to HKEY_CURRENT_USER\Software\Microsoft\Office\x.0\Outlook\AutoDiscover (x.0 corresponds to the Outlook version: 16.0 = Outlook 2016)

For Outlook 2016 version 16.0.6741.2017 and later versions, please add the following

DWORD: ExcludeExplicitO365Endpoint
value 1

We're just testing this now.

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David HaycoxConsultant EngineerCommented:
This fix listed in my post above worked for us.
loosainAuthor Commented:
We found out that it obviously is a problem eith the newest update. We found another way to go back in the update-path:

cd %programfiles%
cd "microsoft shared"
cd clickdown
OfficeC2RClient.exe /update user updatetoversion=16.0.8326.2107
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