
Can you confirm a few things about the Events main form that’s failing to open in the classic form editor?
Does the form have any subgrids? If yes, are any using the Power Apps grid control (modern grid)?
Are there any PCF (custom) controls on fields?
Is there an embedded Canvas App, Web Resource (HTML/JS), or an iFrame on the form?
Those components are common triggers for the classic designer to throw a generic “An error has occurred”, while the same form still opens fine in the modern designer.