A. Yes, all plugins are updated.
B. Does this help?
directly on loading in load-scripts.php
Uncaught TypeError: Cannot read property ‘isArray’ of undefined
at new wp.Backbone.Subviews (load-scripts.php?c=0…p-api&ver=5.0.3:255)
at n.constructor (load-scripts.php?c=0…p-api&ver=5.0.3:255)
at n.constructor (load-scripts.php?c=0…p-api&ver=5.0.3:293)
at new n (load-scripts.php?c=0…p-api&ver=5.0.3:252)
at HTMLDocument.init (load-scripts.php?c=0…p-api&ver=5.0.3:297)
at i (load-scripts.php?c=0…-widget&ver=5.0.3:2)
at Object.fireWith [as resolveWith] (load-scripts.php?c=0…-widget&ver=5.0.3:2)
at Function.ready (load-scripts.php?c=0…-widget&ver=5.0.3:2)
at HTMLDocument.K (load-scripts.php?c=0…-widget&ver=5.0.3:2)
Uncaught TypeError: Cannot read property ‘each’ of undefined
at n.constructFromSchema (load-scripts.php?c=0…p-api&ver=5.0.3:303)
at n.<anonymous> (load-scripts.php?c=0…p-api&ver=5.0.3:303)
at n.<anonymous> (load-scripts.php?c=0…p-api&ver=5.0.3:252)
at n.<anonymous> (load-scripts.php?c=0…p-api&ver=5.0.3:249)
at _ (load-scripts.php?c=0…p-api&ver=5.0.3:252)
at m (load-scripts.php?c=0…p-api&ver=5.0.3:252)
at f (load-scripts.php?c=0…p-api&ver=5.0.3:252)
at n.u.trigger (load-scripts.php?c=0…p-api&ver=5.0.3:252)
at n.set (load-scripts.php?c=0…p-api&ver=5.0.3:252)
at Object.t.success (load-scripts.php?c=0…p-api&ver=5.0.3:252)
After some time:
wp-auth-check.min.js
Uncaught TypeError: Cannot read property ‘hasClass’ of undefined
at HTMLDocument.<anonymous> (wp-auth-check.min.js?ver=5.0.3:1)
at HTMLDocument.dispatch (load-scripts.php?c=0…-widget&ver=5.0.3:3)
at HTMLDocument.r.handle (load-scripts.php?c=0…-widget&ver=5.0.3:3)
at Object.trigger (load-scripts.php?c=0…-widget&ver=5.0.3:3)
at Object.a.event.trigger (load-scripts.php?c=0…-widget&ver=5.0.3:9)
at HTMLDocument.<anonymous> (load-scripts.php?c=0…-widget&ver=5.0.3:3)
at Function.each (load-scripts.php?c=0…-widget&ver=5.0.3:2)
at a.fn.init.each (load-scripts.php?c=0…-widget&ver=5.0.3:2)
at a.fn.init.trigger (load-scripts.php?c=0…-widget&ver=5.0.3:3)
at Object.<anonymous> (heartbeat.min.js?ver=5.0.3:1)