Thanks to help from the Prosody developers, I applied changes to the core Prosody code which fixes SNI. This in turn solves the issue with Google captcha returning an invalid response. Registrations now work once again! This time I’ll be sticking with Google captcha, as other solutions have allowed bots to register tens of thousands of accounts in a quick period of time.
The changes made to fix SNI were pushed to Prosody’s trunk branch, which contains unstable code. Rather than switching entirely to Prosody trunk, I applied the code from the trunk branch only to fix SNI. If you’re curious, the changes came from hg.prosody.im/trunk/rev/d4390c427a66 and hg.prosody.im/trunk/rev/6c804b6b2ca2. The commit I made at GitHub is here as well github.com/unredacted/xmpp.is/commit/dec09989c3164903588cf41b6e7ba4ca3eadf2c9.
I decided to make these changes because of the growing popularity of the server. I also know that the server is used by activists, journalists and people simply looking for a secure and privacy friendly way to chat. This is very important to me, so happy chatting!