Volume Hash Mismatch

Thanks for the update. Bad memory means the Logic Board is bad since the memory is soldered onto the Logic Board. Seems like it is time to retire this laptop.

This is the first report I've seen for this issue on a 2015 model.....all the other reports have been on the USB-C models. In those cases, the Logic Board had to be replaced to resolve the issue, with several people confirming that Apple's service diagnostics reported bad memory.

Your laptop is interesting because the SSD on your laptop is removable & separate from the Logic Board unlike the USB-C models.

Try running First Aid while booted into Recovery Mode to see if the errors can be fixed (doubtful).

Try running the Apple Diagnostics to see if any hardware issues are detected.

You can also perform an extended more thorough memory test by using a bootable Memtest86 USB stick. Test with the default settings. If no errors and if it does not cause a system freeze, then try modifying the Memtest86's CPU settings to "Sequential" and "Round Robin" to see if they trigger any failures.

Run DriveDx (free trial period) to check the health of the SSD. Post the complete DriveDx text report here using the "Additional Text" icon which looks like a piece of paper.

Performing a clean install of macOS by first erasing the SSD will likely fix the problem for a short time, but if there is a hardware issue with the laptop, then expect this Volume Mismatch error to occur again at some point.