2roberto: So, With VAL=2880 VBox starts ok, but you got error -1018? Unfortunately, no fix for this error, so far. On IBM kernels, CPU ID is got as Local APIC ID, which is not guaranteed to be in a contiguous range. So, when you got into the "hole", you got this error. The only means to get rid of this error is currently, install an OS/4 kernel. It can get CPU ID's reliably. (In the future, maybe, some workaround for IBM kernels is possible, but for now, use OS/4 kernel). Also note that if you'll get this error or not, depends on LAPIC ID's in your system. If they are contiguous, all is ok (like on all three of my machines). If not, you can get this error.
VT-x should be disabled in VBox settings, if you have IBM kernel!