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Simple diagram for a failsafe system. If it isn't the probe it must be the VE100 there has been issues in the past with them resetting to a default setting after a power failure I am unsure what the default is but it might be something to check that the set point is still what you set.

If the ambient temps in the room are very cold it may effect the probe and drop it to a much lower temp disproportionally and force the rack to over heat to adjust the cold probe. I am just grasping at straws.