Hi Christian,
I disabled everything except PolyLang and switched browsers
Using Chrome I now have console errors:
JQMIGRATE: Migrate is installed, version 1.4.1
load-scripts.php?c=1&load[]=jquery-core,jquery-migrate,underscore,shortcode,utils,backbone,wp-util,moxiejs,plupload,jquery-ui-core,jquery-ui-widget,jquery-u&load[]=i-mouse,jquery-ui-sortable&ver=5.1.1:3 [Violation] Added synchronous DOM mutation listener to a 'DOMNodeInserted' event. Consider using MutationObserver to make the page more responsive.
add @ load-scripts.php?c=1&load[]=jquery-core,jquery-migrate,underscore,shortcode,utils,backbone,wp-util,moxiejs,plupload,jquery-ui-core,jquery-ui-widget,jquery-u&load[]=i-mouse,jquery-ui-sortable&ver=5.1.1:3
a.event.add @ load-scripts.php?c=1&load[]=jquery-core,jquery-migrate,underscore,shortcode,utils,backbone,wp-util,moxiejs,plupload,jquery-ui-core,jquery-ui-widget,jquery-u&load[]=i-mouse,jquery-ui-sortable&ver=5.1.1:8
(anonymous) @ load-scripts.php?c=1&load[]=jquery-core,jquery-migrate,underscore,shortcode,utils,backbone,wp-util,moxiejs,plupload,jquery-ui-core,jquery-ui-widget,jquery-u&load[]=i-mouse,jquery-ui-sortable&ver=5.1.1:3
each @ load-scripts.php?c=1&load[]=jquery-core,jquery-migrate,underscore,shortcode,utils,backbone,wp-util,moxiejs,plupload,jquery-ui-core,jquery-ui-widget,jquery-u&load[]=i-mouse,jquery-ui-sortable&ver=5.1.1:2
each @ load-scripts.php?c=1&load[]=jquery-core,jquery-migrate,underscore,shortcode,utils,backbone,wp-util,moxiejs,plupload,jquery-ui-core,jquery-ui-widget,jquery-u&load[]=i-mouse,jquery-ui-sortable&ver=5.1.1:2
sa @ load-scripts.php?c=1&load[]=jquery-core,jquery-migrate,underscore,shortcode,utils,backbone,wp-util,moxiejs,plupload,jquery-ui-core,jquery-ui-widget,jquery-u&load[]=i-mouse,jquery-ui-sortable&ver=5.1.1:3
on @ load-scripts.php?c=1&load[]=jquery-core,jquery-migrate,underscore,shortcode,utils,backbone,wp-util,moxiejs,plupload,jquery-ui-core,jquery-ui-widget,jquery-u&load[]=i-mouse,jquery-ui-sortable&ver=5.1.1:3
(anonymous) @ icl_media_manager.js?ver=3.3.9.1:268
27[Violation] Added non-passive event listener to a scroll-blocking <some> event. Consider marking event handler as 'passive' to make the page more responsive. See <URL>
load-scripts.php?c=1&load[]=jquery-core,jquery-migrate,underscore,shortcode,utils,backbone,wp-util,moxiejs,plupload,jquery-ui-core,jquery-ui-widget,jquery-u&load[]=i-mouse,jquery-ui-sortable&ver=5.1.1:2 [Violation] 'DOMContentLoaded' handler took 1103ms
[Violation] Forced reflow while executing JavaScript took 408ms