I was simply updating from 5.3 to 5.3.1 and got the following error on System Status Page -
Sorry, due to an error, we are unable to fulfill your request at the moment. You may want to contact your administrator or service provider with more details about what action you were performing when this occurred. You have requested a non-existent service "civi_flexmailer_required_tokens".
This error replaces the entire System Status page. I resolved it somehow by disabling flexmailer and mosaico and then re-enabling them. Quite odd.
Update: This error also occurred when updating from 5.3.1 to 5.6