How to fix broken Media Editor image preview

A lot of people seem to have problems that the media/image editor preview suddenly stops working, seemingly without cause. I am one of those people. It has happened to me twice, and after surfing these forums I found a lot of suggestions, tried many of them and considered two of them working.

1. Check your functions.php, in main or child theme, for spaces between lines, as;

<?php     ?>

<?php     ?>

It needs to be

<?php     ?>
<?php     ?>

2. Check file encoding in recent changed files, so that it is not UTF-8 with BOM, just plain UTF-8. I edited my wp-config using W7 Notepad through FileZilla and bo(o)m, then the fault came back. Use Notepad++ or similar tool to check and change encoding.

I hope this post helps other users searching these forums for solutions.