I managed to arrange with suphp.conf (the one in the /etc/suphp directory).
I just added a wildcard to the end of the path.
I was forced to "chmoded" (755) /usr/local.
I do not understand why in this installation I was forced to do this manipulations. I installed IspConfig dozens of times, and I never faced this problem.
Remains the consequences of chmod 755 /usr/local: it is not the Debian way for a directory... I do not know what it will produce as a side effect...
Remains in my logs hundreds of lines of this type, but it must be another problem...
[Mon Dec 03 05:30:07 2012] [error] [client xx.xx.xx.xx] PHP Strict Standards: Non-static method SimplePie_Parse_Date::get() should not be called statically, assuming $this from incompatible context in /usr/local/ispconfig/interface/lib/classes/simplepie.inc.php on line 10870, referer: https://mydomain.tld:8080/index.php
If some kind soul has an explanation, (s)he is welcome... (literal translation of a French-speaking)...