I tried to download Quartus Prime this morning but not work.
I tested several hours, the web page always showed "Bad Request
Error parsing headers: 'limit request headers fields size'..
When will this be fixed?
Please clear your browser history,cache & cookies close the browser & then reopen & try.
Check with another browser if you have.
May I know which Quartus edition & version you are looking for?
Recheck with below link,