In a Microsoft Exchange Server 2019 and Office 365 hybrid environment or in an Exchange Server 2016 and Office 365 hybrid environment, on-premises administrator signs in to on-premises Exchange admin center (EAC), and navigates to Office 365 EAC by clicking the Office 365 tab. When the administrator clicks the Enterprise tab to switch back from Office 365 EAC to on-premises EAC, the on-premises EAC page can’t be loaded successfully. This issue occurs if you enable the Google Chrome SameSite Cookie new behavior through the "SameSite by default cookies" flag (chrome://flags/#same-site-by-default-cookies).
To fix this issue, install one of the following updates:
For Exchange Server 2019, install the Cumulative Update 5 for Exchange Server 2019 or a later cumulative update for Exchange Server 2019.For Exchange Server 2016, install the Cumulative Update 16 for Exchange Server 2016 or a later cumulative update for Exchange Server 2016.
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Learn about the terminology that Microsoft uses to describe software updates.