thanks for your investigation, this is typical for windows ;-)
I´ll have to render some footage into the dvd-format for test purposes now and will avoid using wmv.
A rolling back is too difficult - I do always update windows and who knows how far back I would have to go... (I´m sure the "update" is mentioned in the Premiere forum, but it is easier to use another file format in Premiere when exporting videos...)
I´m pleased with the knowledge, that I did nothing wrong, ´cause I was not understanding the problem... but I wonder that there is no update for encore 5.1 solving the problem...