When booting in BIOS mode (not UEFI mode), around one of each three tries Xenserver does not boot because is "Unable to bring up CPUX (error -5)".
This happen only in BIOS mode. In UEFI mode this problem is not present. Unfortunately UEFI has other issues related with iGPU and so we have to stay with BIOS mode.
This problem could be associated with this existing bug (but closed in false) and it appear to be Xen related.