On August 10, 2022, at ~8:44AM EST, Microsoft communicated via tweet (@MSFT365status) that they're investigating an issue where some users in the EMEA region are unable to connect to some Microsoft 365 services.
For IT pros or system admins, additional information can be found under SI MO411804 in the admin center.
Reports indicated this is due to a Cisco Meraki firewall Intrusion Detection and Prevention (IDR) false positive blocking Microsoft 365 connections with "Microsoft Windows IIS denial-of-service attempt" alerts. According to a Cisco Meraki employee, "We would like to make you aware of a vulnerability reported by Microsoft CVE-2022-35748 , triggering SNORT rule 1-60381".
The employee also stated, "Our recommendation at this time is to follow Microsoft's guidance and ensure that your Servers, OS and software are up to date with the latest security patches."