A clear and concise description of what you expected to happen.
The german, splitted zim works. The english, unsplitted zim doesn't. The external sd card has an exFat FS. After selecting the english zim I get the error "ZIM file ...
Then maybe try running 'file' on the library to see if it's in the correct format for your CPU and environment. (eg, make sure it's not on the wrong side of the 32/64-bit divide.) And as ...