OK, this is how to solve it.
Be aware that this is true for an ispconfig working under Fedora 5 - 7. Maybe what i´m going to write works over other operating systems but then, it will be just a lucky coincidence.
Falko had reason, the log was full of information but the info wasn't so old. In the path var/www/webX/log there is a file called error.log. This receives all the error-warning notifications of my web page. As the directive display_errors of your php under ispconfig goes to off, your site won´t show any error-warning if it exists, but the log will be registering them. Here, I must make clear that your web pages may run even when warnings rise up. As this log affects your administrator qouta, while having more visitors, you will reach the qouta faster. Once you are on your limit, you may stop receiving emails if you configured your site to do so.
So what to do ? just go to the path i mentioned in the first paragraph, clean up de log and it will be all. I deleted the error.log file with rm command and I recreated it. The system is working now. Of course I will reach my quota again if i don't fix the code of my page but this will give me a time while I put hands on.
to see the errors - warnings of you page, use the ini_set('display_errors',1); directive. If you are having this problems, you will see on your page the same messages that populate you error.log file.
Thanks a lot to Falko and Till, as usual, excellent support !!!