Can not edit CMS static block

[CasparStraight]

Dear Magento Community,

Today I tried to edit a CMS block but it would just show an empty space as shown below. This used to work without any problems about two months ago, in between this time a few security patches have been installed but that's it. This happens in both Chrome, Firefox and Internet Explorer. Would anyone have an idea of how I could fix this?

Best,

Caspar 

[CasparStraight · Accepted solution]

I sort of fixed it by going to /lib/Varien/Data/Collection and re-uploading the original Filesystem.php (copy obtained from https://github.com/OpenMage/magento-mirror/blob/magento-1.9/lib/Varien/Data/Collection/Filesystem.ph... which was for some reason renamed to Filesystem.php.suspected, probably because we were hacked previously.