I partially solved this issue today by deinstalling LAV Filters and Haali Media Splitter. One of them must have been the root cause. I did not check which one. Elements Organizer 11 and 12 support mts and avi video thumbnails and preview (but without sound), now.
The m2ts videos from my Panasonic camera still do not work. Elements Organizer 12 does not create thumbnails. When initiating the preview Elements Organizer 12 tells me that a codec is missing. This is strange because this product shall natively support m2ts files as well.