Do you have access to your server's error-log? A blank page is typical for a fatal error of the PHP-interpreter. In the log you will find the error message and the line of the script causing trouble. This information should serve as an entry point.

Thanks Alfie,

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

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

Fatal error: Call to a member function generate_math_captcha() on a non-object in /...PATH.../includes/register.inc.php on line 259

After that I changed the Spam settings, and - as I wrote earlier - without CAPTCHA everything works fine.

Are there any configurations which I have to make before running the Math Captcha? What can I try?

Thanks for any clues,
Arthur Dent