Actually, I have to correct myself. It was "Visible Dust Sensor Clean" that I used and that left me some residue. I had bought it in my local photo shop and it cost me 35$...
So there was a B17 that stopped by and I went out to take some pics. I was working on some long exposures again and even though I had the view finder covered I got the same light leak issue again. I also just cleaned the sensor and there was dust after just 50 shots. Pretty frustrated with the camera at the moment!