Thanks for the links. I will try the tips. One question. Why are my harmstrings too tight when my back rounds. I dont feel it in the harmstrings. I thought the problem lies in my back, not in my harmstrings.
@ burton I postet a video of mine squatting.
It looks like you are curving your back when you get to the bottom of your squat. This creates an arch which doesn't have the same support as a completely straight spine. You are also pointing your elbows forward as you move down. Keeping a bar on your shoulders when you practice will help.
The big thing with flexibility in your quads, hamstrings and glutes is it allows a full range of movement when you squat. If you don't have proper flexibility, your body will compensate by your back bending in order to get all the way down.