Fix Exchange 2019 ECP Error

As soon as any administrator successfully signs in to the Exchange 2019 ECP, the page throws a 500 error and nothing else loads. Regular OWA and mail flow are fine; the problem is limited to the ECP portal and affects every admin account we try... (Budget: ₹1500 - ₹12500 INR, Jobs: Active Directory, DNS, IIS, Microsoft Exchange, Network Administration, System Administration, Troubleshooting, Windows Server)