Hi ergo, did you solve your problem? I have some suggestions.
Avoid importing Database to database, try to export data as CVS and then import as CVS into the database checking the fields, very carefully. Remember the "version-sdfasdf" for every imported data, it seems like a dupp almost... and empty the cache from the database.
Recently I discover it is better to do this from the DB (admin) than directly from the "clear cache" under drupal, it missed a lot of things when importing data.
What I mean is creat site from scratch and re create the content types at hand, just do the data import.
I would suggest serving all your data as RSS (full rss) on your old site and then importing the RSS into the new Drupal7 site, it can be done, I did it (other thread) I just had problems with the preview images but that's something different. The FEEDS module can import the images automatically.
I just had an issue, during the days I was doing this over and over there was an upgrade both for the module and core... it never worked again! but I can tell you it can be done. I really prefer my custom CMS made by me :) but if I have to use something else I always go after Drupal, Wordpress is my third option (drupal is the 1st and second) but I always avoid import-export, I always start from scratch creating everything at hand, except from the data import.