From www.bleepingcomputer.com: Proton experienced a significant global outage affecting multiple services, including Proton Mail and Proton Calendar, starting around 10:00 AM ET.

While most services were restored by 1:27 PM ET, users continued to face issues accessing Proton Mail and Calendar until the connectivity was fully restored.

Proton has been investigating the networking issues that led to the outage and apologized for the inconvenience caused to its users.