[jeroenhd]

That error message almost always means someone is using an extremely obscure browser that doesn't pass Cloudflare's not-a-bot-check.

Either the browser has Javascript disabled (Tor), or its Javascript engine is very outdated/broken (a couple of old Firefox forks) or doesn't support Javascript at all.

It doesn't necessarily mean the browser is vulnerable to anything, just that it's not a browser normal websites would expect to encounter so the bot check fails.