I’m having the same issue with adding a repeatable field. In my case the console stated there was an unexpected ;
in or near jQuery.
The event function in scripts.js of the simple-fields plugin requests the field group (repeatable field) from the server using a XHR post request. The respons data has a very confusing line of code in it
if ( ( tinymce.DOM.hasClass( wrapper, 'tmce-active' ) || ! tinyMCEPreInit.qtInit.hasOwnProperty( edId ) ) &:&: ! init.wp_skip_init ) {
I asumed that the &:&:
in the middle of the if statement was the culprit so I added replace('&:&:','&&')
on line 415 in the simple-fields scripts.js which solved the problem.
(Because this forum editor converts any attempt of writing the entity name of ampersand into ampersands I have replaced the comma with a colon in the code above.)
Editing the plugin is of course not ideal but in my opinion better than editing WP core files.