4
The code that Bandit-X posted is in include/cp_functions.php.
1) Try deleting any files in the cache directory.
2) If you still get the error, add the debug code indicated below, and see if the values of XOOPS_CACHE_PATH and $filename look correct:
$filename = XOOPS_CACHE_PATH.'/adminmenu.php';
[color=ff0000]var_dump('XOOPS_CACHE_PATH', XOOPS_CACHE_PATH, 'filename', $filename);#*#DEBUG#[/color]
if ( !$file = fopen($filename, "w") ) {
echo 'failed open file';
return false;
}