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ok. some researches helped me out. i found out that apache is able to create adminmenu.php in the cache folder but is not able to write to it. so the file has 0 byte and the cannot be any adminmenu.
to fix that (well, it's propably not a fix) you create any file in the root-dir with the following content:
include "mainfile.php";
include "include/cp_functions.php";
echo xoops_module_get_admin_menu();
?>
upload it to your ftp and run it. then get the sourcecode of the side and save it to /cache/adminmenu.php
now upload the file. your adminmenu works again ;).
another question: why does that problem occure from one day to the other?