This was on all browsers as the items such as HotTopics, popular headlines and Community Activities didn't load.
For fellow webmasters that might have the same issue and Google for it, I'll describe the issue here and how I solved it.
Issue: After an upgrade to Wordpress 4.3 custom widgets didn't show / disappeared from the Wordpress dynamic sidebar.
Solution: The solution is quite strange, the culprit appears to be somehow database related, the connection between the widgets and the sidebar got lost somewhere. Fixed by going into the Customizer. Then Customize -> Widgets, remove all Widgets and add them again. Save & Publish and they should be back.