Dear All, good news, I nailed it!!!!
This is what I did.
First I used my iPhone with a carpenter surface level tool to check whether or not the bed was level, and it was within 0,1º on all directions. That's quite level isn't it?
Than thanks to Eldrick I used a metal feeler gauge to determine the right distance between the nozzle and the bed in the bedleveling procedure on my Ultimaker.
In the first round I used the 1 mm thick metal feeler gauge and at all the points the nozzle and the bed had a distance of 1 mm, than in the second round I used the 0.8 mm thick metal feeler gauge to determine the right distance between the nozzle and the bed. On all the three points I could slide the feeler between the nozzle and the bed no, touching but no resistance. And than printed the same file again, and it is really square and evenly printed on all sides. So from what I saw during the leveling routine it when using paper you tend to bring the nozzle too close to the bed and that is where the trouble started. Many thanks to Eldrick and I hope that Ultimaker put two feelers instead of the glue in the package when buying an Ultimaker. This saves a lot of time and money and some frustration....