x-mode on "WP Super Cache - PHP Warnings in Apache error.log"

Hi there,
found these warnings in my /var/log/apache2/error.log after installing the WP Super Cache plugin, somebody should fix this:

[error] PHP Warning: fwrite() expects parameter 1 to be resource, boolean given in /var/www/web7/html/ wp-content/plugins/wp-super-cache/wp-cache.php on line 1079
[error] PHP Warning: fclose() expects parameter 1 to be resource, boolean given in /var/www/web7/html/ wp-content/plugins/wp-super-cache/wp-cache.php on line 1080
[error] PHP Warning: fwrite() expects parameter 1 to be resource, boolean given in /var/www/web7/html/ wp-content/plugins/wp-super-cache/wp-cache.php on line 1091
[error] PHP Warning: fclose() expects parameter 1 to be resource, boolean given in /var/www/web7/html/ wp-content/plugins/wp-super-cache/wp-cache.php on line 1092

http://wordpress.org/extend/plugins/wp-super-cache/