Have you any idea about that?

I have the same issue, no matter nginx or apache or without them. As you see, after one goes through invitation link, fills out data and clicks the submit an ajax request is executed to the url (nodebb.zone:4567/register) and stalls for 4 min, then throws an error.

As I noticed, this occurs when the option "Admin Invite Only" is set! Otherwise the registration occurs successfully.