Readers cannot register and log into site

I have a forum on my site that I am trying to get my readers involved in. When the try to register for an account they get this message:

ERROR: The solution of task you submitted was incorrect. Please read the instruction and try again.

What could this possibly mean?

Some of them successfully registered (or already had an account) and left a comment.

My site: https://blog.organicecobaby.com/

Thanks!