A serious problem is any of the following:

• Any permanent loss of stored email contents.

• Any service disruption that lasts for at least 12 hours and results in outbound emails being lost with no bounce notification, or inbound emails being lost in any way.

• Any service disruption that lasts for at least 72 hours and results in users being unable to read emails in their normal client, or unable to send emails.

• A security breach caused by illicit access (not a Swiss court order), which results in private user data being shared without their permission.

• The Swiss government becomes able to legally read email contents.

In all cases the issue must affect at least 1% of all users (or all active users). It also must be ProtonMail's fault, not e.g. those users all happening to have spyware.

I may modify this list to improve it, but not in response to something that just happened.