Sorry for a belated "thanks". Indeed, even Poser 9 is slightly different, adjusting the tumbnail to the same size cured the problem, as well it went away when using the two pane view. But to be eager, I still prefer the the single pane view. 

Oh well, the manual. Believe it or not - I have readed it. I knew about the possibility to adjust the tumbnails as well I was aware about the different views. But I had no idea this could have something to do with the problem.

Inside Poser didn't find an option to reset the warning in the preferencies. But maybe in the related ini.file in Windows. Have to dig deeper into it.