path problem

i have very strange problem, my site seems to run corectly but certain plugins are not working. in debug mode i realized that ther must be something wrong with pathe
p.e.

Warning: include(//wp-content/wp-content/plugins/wp-super-cache/wp-cache-base.php) [function.include]: failed to open stream: No such file or directory in /wp-content/plugins/wp-super-cache/wp-cache.php on line 53

it shows twice wp-content. so the path is wrong and plugin can not find requested files. what is wrong with my instalation?

thanks for advice