And the .acsm files I try to open have been downloaded with three different browsers and looks like
Its a error in the eBook itself. If you get the same Error in diffent browser contact the reseller for the eBook to check the file.
Finally I understood that. So I waited a while, reloaded the books (it was from a library) and it worked. Thx for the answer.
Than the Library are found the error itself.