I hate useragent strings, but with the current state of the Web, your UA sticks out more if you omit the version header rather than copying a common one.

That's still no excuse for a webserver to deny requests by (lack of) UA string, but I doubt that what you're doing is beneficial for any reason.

First I tried it downloading with wget, but it did not work. Ok, maybe they block wget for some particular reason. Then I removed the wget agent, and it did not work either, which is strange. Are you supposed to copy one of those behemoth UA that pretends to be every browser since 20 years ago at the same time?

They probably block commonly-abused user-agent strings such as wget and a blank string. A custom string may work, or just spoofing a browser.

curl works.