After speaking to various USB - (Made in China) only issues had RS232 adapters, because apparently there is a "cloned" Prolific chip was installed always, which is obviously not 100% compatible, I finally bit the "bullet" and I ordered these relatively expensive variant that is also recommended by the chip manufacturers themselves and also with Windows 8.1. should work (I use Windows 7 64-Bit). It worked right away. The matching current drivers have been downloaded immediately upon installation.
The only negative point is the high price in this country as to have in GB / UK at least much cheaper (with the specified high postage costs for shipment to Germany but then ultimately cost the same). Hence the trigger, otherwise it would have been smooth 5 star.