So I opened the box and simply reingeguckt times. The cassette has no mechanism, as one is used to it. It lacks the roles that center the belt and ensure a smoother run. Thus, it is almost obvious that the band can not play. The band has been centered and not by other means, so it is never really in the center and can be easily moved (still in closed cassette).
So far so good. Luckily I had lying around a cassette whose tape was (probably by a car radio) durchgeleiert. This I screwed that swapped tapes and tada: My Cassette Player could be cleaned.
So there is one point (in addition to the duty point) that the product for Bastelfreunde is passable, have so far this lying around a cartridge that can be screwed and can be dispensed with. The cleaning fluid serves its purpose, this is likely but also purchase from other sources.