ISPConfig is just writing a standard vhost, it is not interfering with apache, php and xcache in any way. That somone has the same problem does not mean that its related to ispconfig when its related to apache + xcache and fcgi.
I know but I will be interested to see if someone can make it work with the same config, and by enabling FastCGI in ispconfig panel.
Then ask the xcache developers on how to enable xcache successfully in apache. All successful reports that I found are related to lighty, as xcache belongs to the lighttpd project and php fcgi on apcahe and lighty works a bit differntly (apache does not reuse connections if I remember correctly). So better try eaccellerator or ask the xcache devs on how to configure apache and php-fcgi so that xcache works with apache too. As I told you already above, this problem is not a ispconfig problem. It wold be the same as if you ask how to enable xcache on a Inter processor because your server has a intel processor. ISPConfig does not interact with apache - php-fcgi and xcache, ispconfig just writes a standard apache config file.
Ok, I have tried to contact the xcache devs, but apart from "xcache won't work with fastcgi", all attempts to get something to work have failed. I did find a blogpost (after much searching, too) that tells about FastCGI with a PHP Opcode Cacher. Might this be something interesting for ISPConfig 3? The blogpost is at.. http://www.brandonturner.net/blog/2009/07/fastcgi_with_php_opcode_cache/ Basically it suggests to not use mod_fcgid but to use mod_fastcgi, together with pecl-apc. There are advantages and disadvantages, the blogpost lays them out. Would implementing this be interesting for more ISPConfig-3 users? Please shine all your christmas lights onto this subject.