I have been too stupid and too quick:
Too stupid with my way of creating a hidden folder

.
Too quick with my "all clear"-report. I went to bed in the evening, cheerily looking forward to comfortable reading night. The first ten pages, indeed, were o. k. But then the whole mess came back: Pop-ups, "You son-of-a bitch pulled out the SD-card", "You blockhead didn't format the SD-card in FAT 32", and simply closing the book and returning to the homescreen in a line, just as described in the first post. No two pages could be turned without an issue; it was as if the device wanted to catch up what he had forgotten to grant me three hours before.
So, this afternoon I'll do the test again, hopefully with more intelligence.