Planned this long time ago, needed much time to finish it (due to business work) - but I like the result: My small, long-living blog is now a modern, mobile-first with self-made CSS-Grid Drupal 8 website :-) I've ignored the Microsoft fixes, so you need an updated, modern browser on the desktop to see the sidebar not below but aside the main content.
When upgrading a website from Drupal 6 to Drupal 7 I suddenly saw "<!--break-->" as plain text inside the teaser, instead of hinding the following text - what happened?
htmLawed does its job very well - inside HTML tags there are no comments allowed, so you've to configure it to do so:
Under text formats (/admin/config/content/formats) edit the text format which has this problem and add following in the htmLawed configuration field:
Had this problem today, the solution is rather simple.
Open Phpmyadmin, execute following SQL on your Drupal database:
DELETE FROM menu_links WHERE module = 'system'
Then clear the cache within your Drupal site ("/admin/config/development/performance").
Background: Seems this happens, when you've altered the system menus in Drupal 6.