Unauthorized epic participation for groups you are not a member of
When you mention an epic of a group you are not a member of, you become a participant of that epic with notifications turned on but you get an error when you try to unsubscribe from notifications. This could be related to #7514 (closed).
Steps to reproduce
Go to any epic of a group you are not a member of that you've previously mentioned elsewhere (e.g. in an issue as in gitlab-ce!20154 (comment 84828727)) and try to toggle notifications off.
What is the expected correct behavior?
Probably the user shouldn't be added as a participant in the epic in the first place. Otherwise you should be able to turn notifications off.