Thanks for the reply. I found the fix; of course it's a known problem, and of course it eluded my furious googlings.
https://www.mobileread.com/forums/sh...d.php?t=272327
https://www.mobileread.com/forums/sh...&postcount=739
It seems to be behaving much better now. I kind of wish there was a better overview of the features, though. For instance, is there no battery meter anywhere? And is there some way to go back to the Start menu without restarting the reader?