Thank you all for your replies. All very helpful and very thought through.
After the contact with Sony only gave me a rounabout solution I kept wrecking my brain - and I actually found the problem: ME
As it was my very first voyage into the world of eBooks I couldn't make any sense out of the weird filenames I downloaded. So I renamed them.
After I deleted all the bookfiles (non-Sony Shop) from the eReader and the library and every other place I had put them for save keeping, I downloaded them all over again. Without renaming the files I imported them into the library, connected the reader - and the synch was done in under 10 minutes !!!!
So, it was me after all that.
For all eBook virgins like me out there : DO NOT RENAME THE FILES