OK, this does not seem to be an issue with the plugin. My site log shows this error:
/var/www/vhosts/[removed]/httpdocs/wp-content/plugins/humancaptcha/outerbridge-humancaptcha.php on line 394
[Sun Mar 31 17:13:19 2013] [warn] mod_fcgid: stderr: PHP Warning: Unknown: open(/var/lib/php/session/sess_6r9rm26d61bi6imrkuu1guesf4, O_RDWR) failed: Permission denied (13) in Unknown on line 0
[Sun Mar 31 17:13:19 2013] [warn] mod_fcgid: stderr: PHP Warning: Unknown: Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/var/lib/php/session) in Unknown on line 0
line 394 has session_start();
So clearly the server configuration is wrong. Might be because of the upgrade to 3.5.1, might be because I’ve change the php mode to FastCGI.
Anyhow, the host fixed this.