You also need to make sure the /app/cache directory is writable by whichever user your system is calling the PHP files with.
And Symfony is a pretty powerful framework. If things go wrong though, and you don’t know where to look or have no idea what is breaking and why, then it is like looking at Mount Everest for the first time and thinking now you’ll have to climb it, even though you really don’t want to. That is the downside of such frameworks. They are magical, when everything works right, and downright massively complex and mysterious, when things go wrong.
Scott