MD3200i with R620 - Failed Upgrade to ESXi 6.5 from 5.5

I have the referenced setup. The MD3200i is only accessed via the R620 and is setup as one single RAID 6 virtual disk. (10.91TB)

ESXi is installed on the sd cards on the R620.

Management software version 10.84.G6.58 and RAID Controller Firmware 07.84.53.60

ESXi is currently still on 5.5. I'm trying to upgrade to 6.5U3.

If I do this it essentially fails. The boot takes forever and constantly connects and then timeouts accessing the MD3200i. 

The vmkernel log is full of various errors like:

warning nmp nmp_devicerequestfastdeviceprobe

warning: iscsci_vmk iscsi start connection

The MD appears to go online/offline/times out. I have tried this: https://kb.vmware.com/s/article/1016106 to no avail.

I've disabled IPv6, once  booted into 6.5 I can't browse the datastore via CLI (UI access never works)

Even in 5.5 rescanning my HBAs actually freezes the host for 5 minutes or so. There are no bad HDDs and MDSM says the MD is Optimal.

Is there perhaps a bug in my firmware version (I realize I'm quite behind)? Maybe my configuration? A better way to setup the MD for 6.5 compatibility?

Thanks in advance for any ideas.