Resolved it myself. My original video files are MP4 and I convert them to AVI as Magix won't let me view them as thumbnails or apply effects or do simple crossfades with the MP4s. My converter is ANY Video and the newer versions use a codec that makes the thumbnails in the Magix Media Pool green. I simply uninstalled the video converter and reinstalled an older version of it and the thumbnails now appear as normal. I hope this fix is found useful.
I'm not using the 2015 version, as I mentioned, I'm using Edit Pro 15 Plus and I can't do transitions or apply effects to MP4s as well as not being able to see them as thumbnails in the Media Pool. That is no big problem as I convert them to AVI files which worked fine until I updated my video converter and so reverting to an older video converter was the solution.