Try to switch to the new Exchange admin center manually, then close browser and reopen, check whether you could open the new Exchange admin center from Office 365.
If it still redirects to the old admin center, I suggest you have a check in the development mode, check whether it is linked with the new admin center "https://admin.exchange.microsoft.com/":
If it points to the new admin center link, it means there exists another configuration on your computer to redirect this link to old Exchange admin center. Please try to access from another computer to double check whether it is related to computer configuration.
If it points to the old admin center, you could open a service request to check with Office 365 team.
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