Error composing message after customizing login

[pyro_fire]

Goodnight.

Devs, I needed (for a very specific situation) to change the login authentication of a Roundcube installation.
Basically I changed the LOGIN action so instead of logging directly into Roundcube I consult an external URL and if the user is valid I then request the $RCMAIL->login() function.
Apparently everything worked normally, however when going to compose a new message (?_task=mail&_mbox=INBOX&_action=compose) I get an error accessing some URL via cURL.
Can you identify which URL Roundcube requests when composing the new message?

Thanks.

[SKaero]

Did you customize the login via a plugin or editing the core code directly.