I was able to get an Mbin dev server set up and can confirm it's nothing on our end:
<my IP address> - - [10/Oct/2025:18:55:06 +0200] "GET /u/i_am_not_a_robot HTTP/2.0" 200 1133 "-" "Mbin/1.8.4 (+https://localhost:8443/agent)"
I couldn't tell you what's actually going wrong in Mbin as the server isn't actually processing the requested object.