From the schematic - the humidifier plugs in directly to 120v. I see a 24 volt transformer - that is connected to the humidifer, via the terminal strip in the humidistat.

It looks to me that the humidistat outputs H H must always be supplying 24 volts. Looks like the Humidstat is causing the problem

Here is a link that might be useful: 700 Schematic

The lower schematic - the M (manual) model with a current sensing realy (just senses magnetic field when current it flowing through the wire). If you have this setup, then I'd say the current sensing relay is faulty.