The following refers to the threads
The solution in my case was to uncheck under "Website Administration --> Server --> HTTP" the option "use slash arguments".
Now I have two questions:
1. I am positive that I did not activate this option. It is activated by default anyway. How is it possible that suddenly this option has to be deactivated in order for downloads to work?
2. What is the purpose of this option?
Answers would be greatly appreciated.