At first I didn’t really want take a deeper look at the browser idea because I already wrote in my post that I tried several browsers. But you never know … it can be due to some weird things as you said.
And see there: The cause of the error was actually the browser!
Thank you so much @tobiaseigen !!! You were absolutely right.
Thanks to your suggestion I was able to figure out what caused the error!
for whatever reason the browser I tested (ungoogle chromium, brave, firefox, librewolf, tor browser) don’t start the download if you click the link in your mail inbox in the browser. only if I copy the download-link, open manually a new blank tab, paste the download-link and execute the link manually, then the download starts like there was never a problem 
maybe my mail provider is doing some bllsht link protection like microsoft with their safelinks and before I can initiate the download they open the link, but then many other users should have that problem too. maybe its because of a browser update, but brave is chromium based and all the other are firefox based, that would be a big coincidence if both projects would have changed something same at the same time.
it’s fine for me to copy and paste the download link in the browser manually. maybe one day it will work like before. in any case I have to be logged in in the forum, to be able to start the download, but that makes sense.