Stray voltage on all earthed and unearthed objects
My suspicion would be a broken or high-resistance neutral somewhere in a supply cable.  That would divert return currents through the earth connection instead.  Since earth connections are never zero resistance, that could produce a significant voltage on the earth.  If the neutral at your installation is significantly far from 0V, then that's probably it.

If you can, measure all voltages relative to an independent earth rod, hammered into the ground as far as possible away from any other earth rods or mats.