It looks like you're in the DISA Patch repo which is the latest baseline but not the latest versions. I personally just upgraded our HBSS from 4.6.9 -> 5.1.3 which is the latest DISA supported. There is a newer 5.3.1 but there are no STIGS for it. You can get the latest 5.1.3 from the MIL Forge website.
Did you just do a straightforward upgrade from 4.6.9 to 5.1.3? Any pre-requisites to follow before I upgrade my 4.6.8 to 5.1.3?
I would read over the ePO Upgrade Guide document you showed in the list of fixes above.
There were two issues.
The biggest issue is DB size. The installer complains about your system being too large since we have to store so much data. For where I'm at I only need 365 days of active data so when you run the installer if references a KB article on how to remove old data so I did that but instead of the 30 days the KB references to 1 year. If you've never done this before it can take hours to run.
The second issue is some of the plugins in HBSS do not support ePO 5.x so you need to go through the list of what needs to be updated. Some of them like OAM, APS, and HCEE you have to actually wait until after upgrading to ePO 5.x to install.
I highly recommend doing this on a backup of your HBSS system first because since our environments are so specific anything could go wrong.