After a recent migration from Drupal 7 to Drupal 8 I encountered this error. "Fix: This block is broken or missing. You may be missing content or you might need to enable the original module. " It’s not a bad error necessarily, but it certainly was annoying enough that something had to be done to find out what was causing it. I was getting this error in the Seven theme used in administration view, but it could very well appear anywhere technically in any part of the site where a broken block is placed. Essentially this is a broken block that is still placed in a location after the migration.
Head over to the block lay out for the theme, in my case it was:
/admin/structure/block/list/seven
Look down the list of blocks, and look for any that say missing/broken block and remove them. It is that simple. I hope this helps somebody out getting this issue fixed.