Calc check failure, reCaptcha works OK

Hello, I installed the plug-in, it all works fine. Even translated the language file in Czech. Only - when somebody fills in a wrong entry in the spam check, the page does not redirect to the form again, but a white page appears. When clicking on refresh (F5), the right page ("You entered a wrong spam check") appears.

I searched the forum, bit could not find an answer to this (and my German is not so good, so I cannot search so well in that language - so I may have missed an entry in German on this topic)

Joomla 1.5.9, VM 1.1.6 stable

Anybody an idea where to look for the cause?

The site is www.reggaeshop.cz

(I disable the plugin now, since I do not want users to get confused or annoyed)

Thanks,

Peter

Added:
I experimented with it a bit more. The result:
- When disabling "Keep form data on fail?", the page loads normally
- However, the next error is, the :always fail spam check" error
- Also: Even while I have set "use words instead of numbers", it still ery now and then shows the number instead of the words

When using reCaptcha instead of calc check, it works fine (pass/fail spam check, etc), only the issue with not keeping the form data (having to set it like this because of the earlier mentioned "white page" error) remains.

I now kept the reCaptcha, but still would prefer the calc check - since i think this protects better against human spammers (who, probably, do not know Czech - I translated the ini file into Czech)

Any help appreciated! Thanks!