[LucyOne]

Thanks, chrox!

Apart from the problem opening epub files, I also noticed this: When pressing the home button on the device while reading in Koreader, the home screen is shown for a second, then flashes and shows the book cover. When touching the cover, an action of the home screen is executed, e.g. a book is opened or the library.
Then the home screen is shown again when I press the menu button, but sometimes the status bar still shows the book cover.

Edit: These problems seem to be solved with the latest build.