Trouble installing wp supercache

Hi.

I tried to install wp-super-cache. I think I followed all the steps correctly. Here is the error I get after enabling “WP Cache and Super Cache” in the options of WP Super Cache :

Warning: sem_get() [function.sem-get]: failed for key 0x152b: Permission denied in /var/www/html/xxxxxxxx/wp-content/plugins/wp-super-cache/wp-cache-phase2.php on line 100

supercache plugin version : 0.5.1
wordpress version : 2.3.1
themes installed : BloggingPro Theme – Widgets Ready 1.2
other plugins : most commented, recent comments and recent posts

When I access a page for the first time (the main page of the blog for example), I see the error message, When I’m in the admin of wordpress, I always see the error message. When I access a page, a file (or some files) is created in the cache folder, but in the admin of WP supercache, there is “0 cached pages” for WP-Super-Cache but 2 for WP-Cache.

I tried chmod 777 on the cache directory
I tried deleting the cache directory
PHP safe mode is off
I deleted the wp-cache var in wp-config
I tried with php 4.3.10 and I upgraded to 5.2.0, still the same behavior.

Any idea?