Ha! Today the admins found the cause of my always dropping network connection recently:
It seems that the network-cable, which is from my computer to the network socket. Replaced cable, network fine! Finally.
Sad reason why I'm working a bit longer today, a co-worker is leaving company soon because (I think) there is no chance for him to climb ub the career ladder any further without kicking out someone.
This can become a more challenging future for us, because with him leaves very much knowledge and experience, but there are also opportunities for other people. His work has to be done by someone.
Nevertheless, I wish him only the best for his future. Good Luck, W.