Repeated "859: unexpected token" exceptions in a Rails 6 application

Over the last several weeks, my Rails 6.1 app has been raising hundreds of exceptions like the following:

An ActionDispatch::Http::Parameters::ParseError occurred in regdevs#create:
859: unexpected token at 
'utf8=%E2%9C%93&authenticity_token=fnrg63qtelQvkQx9NQx8SaZIp7mR500toEiWdaJe91
%2FOzsEvRbGD4Ow6NNADCtjw7H3EkDvFZVGP4gQkGZceEw%3D%3D&user%5Bemail%5D=jrmills91
%40hotmail.com&user%5Bfirst_name%5D=wJAhzNnPWbqMdfH&user%5Blast_name%5D=tKdp
IVXJlneZbiuA&user%5Bnickname%5D=lBdbyYgiaHt&user%5Bpassword%5D=0SgLpVuf5Wqs
%21&user%5Bpassword_confirmation%5D=0SgLpVuf5Wqs%21&user%5Bterms_of_service
%5D=1&commit=Complete+registration&g-recaptcha-response=dzghTWcBpmI'

The error occurs when "someone" tries to register for a new account (via Devise) with bogus credentials. It clearly looks like some sort of attack.

I have two questions:

1. How can I find out more about this specific 859 unexpected token error?
2. Should I be concerned? The recaptcha guard seems effective in blocking account creation, but I wonder if I am missing something