Suddenly the admin has no longer access to index.php and admin/index.php (and spin-off pages..)

I saw a strange effect that the default category was createde several times (10x)

I traced down admin/index.php:

first stop was at line $adminroot =& admin_get_root();

If I make $adminroot=1; then the next stop is:

admin_externalpage_setup('adminnotifications');
admin_externalpage_print_header();

Any idea?