You could be right. I fiddled somewhat with several files and it still worked, but ... after some time I did a shutdown/restart of the reader and the cut-off appeared again. To be on the safe side I decided to replace the CSS file again which did not help. Only after doing a reader shutdown/restart again, all was working again.
I figure, as I did completely removed the original CSS, that the reader somehow "remembers" the CSS settings (perhaps after sending new e-books to the reader), only to try to re-read it after a shutdown/restart and/or when new e-books are send to it. In those cases it could not find the setting anymore (CSS was deleted), hence resulting in cut-off again of characters.
Well, all works now again, so we can conclude I was perhaps too hasty by removing the CSS
