@trekk: writing down the few lines above I started to think why I need the third line (my own user data!) for the series thing. I had started to setup a series metadata approach long time ago when calibre had no solution for this - and when calibre's own series solution was released: I forgot about that user column because I never needed it any more. Shame on me!!!
Now: I deleted that user defined variable and updated the series titles metadata with the new calibre setup. Alas - now I have a correct display in the IP3.
All my other eReaders never complained about that additional line. remains the question why the IP3 is parsing that specific addtl line.
Thanks to you (and all readers) for being patient with me
Klaus