thanks for your help so far.
I found out that actually my mobilephone (sony ericsson w810i) can also read ebooks with a little programm named "MText Reader 2" and it works. It also seems to work with a lot of other mobilephones, too (maybe this could help you too, squeek? I mean, if we don't get our players to work with ebooks.)
I could now say, okay, I can read ebooks, let's end the thing with my mp4-player and just hear mp3s and watch movies on it. But unfortunately you cannot adjust the time until it goes to standby on my phone, so I have to use a bigger font and am typing my fingers down to the bone, because I have to turn the page every two or three seconds...

My mp4-player instead has a much bigger screen and adjusting the standy-time works just fine. So I would be really much happier, reading my ebooks on it instead of on my mobile phone.
@ sqeeek: thanks *lol* I'm relieved now
@ admin: I tried the demo-txt-file but unfortunately it does not work just like the rest of the txt-files I used.
But could you tell me how your ebook-folder is named or better: how it was ORIGINALLY named when you bought it? I don't really think that this could be the problem, but, well, who knows?
Because on my player there wasn't a special folder and also no demo-file (maybe the seller deleted them?).