So, the problem is that Feedreader does not open the articles to the frame so you could read them. Only a blank space with the buttons "open in browser" and such. No text from the article or anything, I also didn't find anything in the preferences and reinstalling didn't help. What could cause this? Thank you.
I got feedreader working
I got feedreader working again with a surprisingly simple solution though it might not work for all. I uninstalled feedreader to reinstall it and instead of downloading feedreader installer, I downloaded the ZIP package of it, unzipped to Program Files and started it. From that moment on it has worked perfectly.