It seems Liip Imagine Bundle has added a new setting config in the past days. This setting theorically allow you to add a post processors to the generate images for optimizing, for example. See this pr
https://github.com/liip/LiipImagineBundle/commit/69b3921af8f4d0c12faa83e2449080934477eca0 and doc

After this was added to liip and updated to a ez5 install, ez5 can't generate new images anymore. when ez5 needs to create a new image a exception is thrown saying

An exception has been thrown during the rendering of a template ("Notice: Undefined index: post_processors in [...]/vendor/liip/imagine-bundle/Liip/ImagineBundle/Imagine/Filter/FilterManager.php line 144")