Hi All,
Thanks for your input on this, I had exactly the same issue upgrading from 5.1U2 to 5.5U1:
Custom ISO, altering drivers and such sounded plausible but thought I'd try the easier option first and it worked; I downloaded the March release of the HP ESXi 5.5U1 image that was removed from general release and replaced with the June/July release as the March one had the Heartbleed vulnerability.
I installed the March release via update manager, applied all updates after reboot, now I'm up to speed and on a newer build than the ISO that was giving me the errors to start with.
Found the March release here: http://ddl1.digiboy.ir/vmware/5.5/VMware-ESXi-5.5.0-Update1-1623387-HP-5.73.17-Mar2014.iso
Not my site but the MD5 checksums check out and it has worked fine for me!