Hm, how frustrating. I've seen that issue before on my own VPN and I think it's something to do with the interaction between Cloudflare's security cookie and my site, but it's tricky to reproduce. I'm trying it using the Tor Browser now and it seems to be working.
If all else fails you can try https://sgentle.github.io/scrawl/
I tried to solve the captcha several times, before giving up. This will happen to all users that try to visit your site using Tor.