During the development I installed some additional packages to Apache. Mostly it's easy to spot if you check the log files for errors, but for plugin sfImageTransformPlugin I use the mime type detector and in order to use it, the mime type package has to be installed. The result was blank page when manipulating images, which happened when initializing the sfImage class. To enable the detecting mime type I configured app.yml
sfImageTransformPlugin: mime_type: auto_detect: true library: MIME_Type