The ground of my PCBs (0V negative from the PSU) and the PSU case are both connected to the mains earth.
They are housed in an ABS box with screws visible on the outside so the whole lot needed to be earthed.

There is another one of these blue ceramic caps inside the PSU that goes from the negative out to case ground. I'm guessing this is the MOV you mentioned.
This is essentially bypassed by the earth wire I have running between case ground and PCB ground.

The "0V" is the negative output of the PSU.

I've got some 0.25W 1MOhm resistors that I could string together as a test.

Someone I work with has suggested the blue cap between the rectified mains and negative output may be failing.
This PSU has been on 24/7 for months without a problem but I know caps will fail eventually.
I've got several other working supplies and some that have never been turned on yet so I can try swapping out small components like that.