boost_cache_clear_all does not delete files

If I'm right, the function boost_cache_clear_all should delete all files from my cache directory if I manually clear the cache.
This doesnt happen and as I have CSS aggregation enabled the anonymous visitors to my site get a cached page, but with a wrong CSS-File resulting in a bad design.
The files are furthermore never cleared using the clear cache-command on the admin => settings => performance page, although they have expired.

On the other hand, a cronrun clears all expired files from the cache-directory, but also fails with clearing still unexpired files.

While looking at the code, I didn't notice any severe difference regarding the boost_cache_clear_all and the boost_cron function.
I also tried to remove the NULL-parameter in the cache_clear_all function, without success.

Has anybody the same problem, or some hints?

Specifications:
PHP 5.2.6
Boostversion 6.x-1.x-dev
Drupal 6.8
Installation in subdirectory, therefore some modifications to the .htaccess were made, but I highly doubt, that this is the problem...