What does the iLO / IML logs show for the memory ? Any issues or are they all active and healthy.
The only other thing I can suggest is to try and install the HPE Customized ESXi ISO . It includes drivers from the vendor. I've always used those images on my G9-G10 servers..
Also check the hardware compatibility guide to make sure your G6 is actually supported by this version of ESXi.
I hope some of this helps you find the cause. Good luck !