There is actually an error in the browser console, which is related to Simple Tags:
helper-suggested-tags.js?ver=2.5.1:59 Uncaught TypeError: Cannot read property 'getEditedPostAttribute' of undefined
at getContentFromEditor (helper-suggested-tags.js?ver=2.5.1:59)
at HTMLAnchorElement.<anonymous> (helper-suggested-tags.js?ver=2.5.1:12)
at HTMLAnchorElement.dispatch (load-scripts.php?c=1&load[]=jquery-core,jquery-migrate,utils,jquery-ui-core,jquery-ui-widget,jquery-ui-mouse,jquery-ui-position&ver=5.1.1:3)
at HTMLAnchorElement.r.handle (load-scripts.php?c=1&load[]=jquery-core,jquery-migrate,utils,jquery-ui-core,jquery-ui-widget,jquery-ui-mouse,jquery-ui-position&ver=5.1.1:3)
getContentFromEditor @ helper-suggested-tags.js?ver=2.5.1:59
(anonymous) @ helper-suggested-tags.js?ver=2.5.1:12
dispatch @ load-scripts.php?c=1&load[]=jquery-core,jquery-migrate,utils,jquery-ui-core,jquery-ui-widget,jquery-ui-mouse,jquery-ui-position&ver=5.1.1:3
r.handle @ load-scripts.php?c=1&load[]=jquery-core,jquery-migrate,utils,jquery-ui-core,jquery-ui-widget,jquery-ui-mouse,jquery-ui-position&ver=5.1.1:3