unfortunately I don't have access to the error log. I just saw (by chance) a browser window with following lines:

BTW, a server must not be configured to come up with error-messages to the client.

Notice: Undefined variable:captcha in /...PATH.../includes/register.inc.php on line 259

OK this is the call to the function generate_math_captcha() in captcha.php.

Stupid question: is the folder /...PATH.../modules/captcha/ (and all it's contents) available on the server?
Maybe you just unzip the forum again locally and upload only this folder again.