Have you checked if your HW is supported by 5.5? Even if it was supported by 5.1 it does NOT mean automatically it is supported by 5.5. So maybe 5.5-installer does not know how to continue. I have hit this obstacle myself: my quite new and supported (up to 5.1u1) Intel server-board for xeon/sandy cpu was suddenly dropped from HCL for ESXi 5.5...