SSRN "unusual download pattern"
Such sites often have measures against crawlers. Sometimes normal users can trigger them. For example some university institutes which have a single IP address for everybody at the institute can even get blocked permanently until someone mails the site admin and explains the situation, which may get the IP whitelisted.

When it is triggered by you alone, probably the best choice is either to slow down and wait a bit between requesting stuff, reconnect your internet connection to get a new IP, or to accept the suggested solution and use a free account to prove them you're human and not a crawler.

Such blocks are most the time IP based. If cookies help you at all, you should NOT delete them, so the site can track you as individual user.

If deleting cookies would help, crawlers would just delete them (or not store them at all), so the site would have no benefit to allow access after you deleted the cookies. If changing the user agent would help, crawlers would just randomize the user agent string. So you can be pretty sure, that such things will not help you.