Event Attribute Definition Aspect "Event Type" Selection
Please download this bulletin from ABB Library and study the workaround suggested.
800xA Base 5.x Creating an attribute extension to an event category can cause failures in the Alarm system, 3BSE064359.
IIRC, the important setting is to a) enable Extension checkbox, b) select the category that shall be extended.
The below image describe extending the AC 800M process alarm category "Process Condition Event" - notice the greyed out checkboxes.
This should work for you too.