Just noticed something strange: I transferred two *.cbz files into gallery, and decompressed them. One was working fine, the other not. After a bit of testing, I found out, that the "#" in the filename was the problem. The decompressing and renaming was okay, but when starting the folder with the # in the folder name, I got the error message, that there is no image numbered 0. Just to let you all know.
And thanks again to silver18 for making a good app even better!