The negative effects of an UV filter

Lately I had some trouble with my mid-class 70 - 300 mm Canon zoom. For some unknown reason the lens did not communicate with the body.

We went on a trip and I borrowed Christina's 75 - 300 mm lens (no image stabilization and a lot simpler). The results shocked me. They were so much better....

Today I fixed my lens and made some shots, toggling between stabilization method 1 and 2 to make sure I took the UV filter off.
The lesson: Why did I ever screw a 15 dollar Chinese UV filter on a 650 dollar lens???
Without the filter my lens is perfect.See the photo's. The first two are my lens without filter. The second two Christina's lens, and no filter. The last two are my lens with filter. I hope the images are large enough for you to see the difference.