Not sure if this helps but I noticed that clicking on the thread's title that links to the url that ends with "unread" loads really slow, vs if you click the last page, for example:
When we migrated to the new server, every single post got marked as unread for everyone. By default XenForo alerts users of a new reply to a thread they're watching only if they've viewed the thread since the last alert.
I tried all captchas both on mobile and pc and they work for me. Regardless, emails are currently disabled until we fix them.