Finally solved it!!! It took me a very very long time to figure this challenge out. But I did it all with pen&paper and Word&Excel. So it is indeed possible to solve it without any computer programming.
Although this is a difficult challenge with a lot of unforeseen twists, I recommend to those who really want to test their skills to try this with Pen & Paper as Viktor recommends. Because even if programming skills are useful and necessary to become a cryptographer, more skills are required to become a good cryptographer, such as thinking out of the box, learning from mistakes and persistence. You need to cope with frustration and have the courage to continue. With this challenge you can test a lot of your capabilities. Don´t give up.
@ Viktor: a very clever challenge, indeed nasty, but a good job [HTML_REMOVED]