Problem with default style after using AP_Languages_and_styles

I used russian modification of punBB in which is already included in plugin a AP_Languages_and_styles. At first I at options changed style to mercury, but then later down some styles and started looking on them in plugin AP_Languages_and_styles.
I choosed to keep VAKA style. But then I noticed that I cant change default style:
----------
File: /www/www.islammuslim.lv/htdocs-forum/include/cache.php
Line: 91

PunBB reported: Unable to write configuration cache file to cache directory. Please make sure PHP has write access to the directory 'cache'
----------

So while I have VAKA for login members, for those who have not logged yet, is default - mercury...

What should I do?