I am installing a Honeywell RTH6580WF with a TACO SR504
I had the exact same problem.  The 24volt term seemed to test 24 volts with the red wire and the COM tested 24 with the white.  It was wired with thermostat wires at Taco right red and left white.  4 thermostats and 2 days later the only solution was.  Wire all "common" wires to the COM terminal and reverse white and red on thermostat.  I did not want to rewire the TACO.  This worked perfect.