@aristath I have the same problem and it seems that this comes from the last update of The events calendar and more precisely from their custom embedded select2 script (See error log) :
select2.min.js?ver=4.6.3:21 Uncaught Error: Option 'data' is not allowed for Select2 when attached to a <select> element.
at String.<anonymous> (select2.min.js?ver=4.6.3:21)
at Function.each (load-scripts.php?c=1&load[]=jquery-core,jquery-migrate,underscore,customize-base,wp-a11y,wp-util,customize-controls,jquery-ui-core,jquery-ui-widget,jquery-u&load[]=i-mouse,jquery-ui-sortable,jquery-ui-draggable,jquery-ui-droppable,backbone,wp-backbone,shortcode,utils,media-models,plupload,wp&load[]=-plupload,mediaelement,wp-mediaelement,media-views,media-editor,media-audiovideo,wp-ajax-response,jquery-color,wp-lists,postbox,&load[]=nav-menu&ver=4.8.3:2)
at i.prepareOpts (select2.min.js?ver=4.6.3:21)
at i.prepareOpts (select2.min.js?ver=4.6.3:22)
at init (select2.min.js?ver=4.6.3:21)
at HTMLSelectElement.<anonymous> (select2.min.js?ver=4.6.3:22)
at Function.each (load-scripts.php?c=1&load[]=jquery-core,jquery-migrate,underscore,customize-base,wp-a11y,wp-util,customize-controls,jquery-ui-core,jquery-ui-widget,jquery-u&load[]=i-mouse,jquery-ui-sortable,jquery-ui-draggable,jquery-ui-droppable,backbone,wp-backbone,shortcode,utils,media-models,plupload,wp&load[]=-plupload,mediaelement,wp-mediaelement,media-views,media-editor,media-audiovideo,wp-ajax-response,jquery-color,wp-lists,postbox,&load[]=nav-menu&ver=4.8.3:2)
at a.fn.init.each (load-scripts.php?c=1&load[]=jquery-core,jquery-migrate,underscore,customize-base,wp-a11y,wp-util,customize-controls,jquery-ui-core,jquery-ui-widget,jquery-u&load[]=i-mouse,jquery-ui-sortable,jquery-ui-draggable,jquery-ui-droppable,backbone,wp-backbone,shortcode,utils,media-models,plupload,wp&load[]=-plupload,mediaelement,wp-mediaelement,media-views,media-editor,media-audiovideo,wp-ajax-response,jquery-color,wp-lists,postbox,&load[]=nav-menu&ver=4.8.3:2)
at a.fn.init.e.fn.select2 (select2.min.js?ver=4.6.3:22)
at f.renderFontSelector (typography.js?ver=4.8.3:144)
So on customizer screen two versions of Select2 are loaded… Removing The Events Calendar script removes the error !