Hi again,
I was finally able to resolve my slow upload network speed problem. It seems that a network prioritization program (ASUS Network iControl) bundled with my motherboard somehow started causing the problem maybe in conjunction with the latest Windows 10 build update I mentioned above. The funny thing is I tried to completely uninstall it, because I never use it anyway, and this broke my networking altogether. Only after the successful reinstallation of the ASUS Network iControl program (I had a couple of unsuccessful attempts too!!!) was my networking up and running again and the slow upload problem magically disappeared at last. Of course now I'm stuck with this program which caused me the problem in the first place, I don't use it, but can't uninstall it either. Thanks a lot for your time and effort to help me resolve this issue.
Sincerely,
Dimitris