Actually i had that problem too, after reading how mon51 needs to be set up. I saw the interrupt table was revectored to 0x8000h.

If you change in your target settings that the interrupt vector table is at that adress (or at the adress set up in your version of mon51) and it should work. At least it did for me.

But some things still dont work like trace buffer and trace cache (the options become visible during uploading but become greyed when running).