So, just type the question and the solution will appear ;)
I checked the SQL permissions, and that's it:
The user, that i use the acces the DB, was created by Confixx (3). It does NOT have the privilege "CREATE TEMPORARY TABLES".
After i granted this, the search works.
It seems, as this privilege is new to MySQL 4, at least it does not appear in phpMyAdmin for my MySQL 3 server. See also then last posts at http://ez.no/community/forum/general/searching