email handlers do not send email

Which version of Webform was working?

Also turn off JavaScript aggregation and get the stack trace.

If you are referring to the Advanced CSS / JS Aggregation Module, I do not have it installed.

Unless there is a setting in the Core modules?

Yes, and please answer all questions.

How do I get the stack trace, I cannot see any option in the Development Module, or is it to be found elsewhere?

It’s in the web console. You said there is a JavaScript error.