Thanks a lot to everybody! daveemtb provided the most easy solution - yes, when working like, my waypoints show up. Great, thanks a lot! In addition to this, I will also look into the Wiki text mentioned by destructor ...

Update: I did a last try and uploaded exactly the same track once again (trial #5). Now, finally, after 4 refusals with incomprehensible "error messages", it was accepted. -> OSM should be more careful before applying intermediate builds on the fly, containing such annoying bugs...

Now I received exactly the same error 4(!) times already this evening and tonight (June 07/08 - before and past midnight), it's most annoying and now I don't feel like contributing any more, although it used to be fun. I have a Garmin GPSMAP 60CSX, I walk along the roads, transfer the recorded tracks with Garmin Mapsource to my PC and upload them to OSM. This worked fine and without any problems during the past 2 weeks (when I became OSM member). But since today this does not seem to work any more, instead I receive error messages with exactly the same "error message" as you do, lacking any reasonable explanation. No, OSM, thanks, I loved your idea but this is too much.