Finally found it:

Source said:

/* Seatbelt the KDC */
if (0 != sandbox_init(argv[0], SANDBOX_NAMED, &errorbuf)) {
fprintf(stderr, "%s: failed to initialize sandbox: %s", argv[0], errorbuf);
sandbox_free_error(errorbuf);
exit(1);
}

...so my bell rang and i exchanged seatbelt from 10.5.5 <> 10.5.6 that it was.
after initialising seatbelt.kext the error stopped!
dmg mount still ok!
so i might close this!
thanx for your help anyway!