Issue solved. It was not the event handling. jsonFormer interprets spaces as the begin of a new field.

This caused the error of the page.

Replacing the spaces by underscores in the cpp source e.g. obj_config[F("Average Period") to obj_config[F("Average_Period") helped.

Lesson learned. Do not use spaces when applying jsonFormer.jquery.js

Inspector of MS-Edge gives more details than Firefox inspector.