Novice
Check the humidity
The humidifier should only turn on if the humidity is below a set level. Make sure the humidity is actually below this level.
Clean the inlet
Check the inlet (where the water comes in), and make sure it's not all gunked up.
Check the valve
If the humidifier supply line connects to the water supply via a saddle valve, close and reopen the valve (and consider replacing the valve, which will require shutting off the water since the valve pokes a hole in the pipe).
Set the humidistat
If a humidistat is used, make sure it's set properly.
Intermediate
Check for water at the solenoid
Close the valve supplying the humidifier, and remove the supply tube from the supply side of the solenoid. Open the valve just a bit, and make sure water is coming from the tube.
TIP: Wrap a rag/towel around the open end of the hose, and hold it tight when opening the valve. The rag/towel will prevent the water from spraying out the end of the tube. Also, don't forget to hold the rag/towel wrapped tube over a bucket.
Check for power at the humidistat
Open up the humidistat, and use a multimeter/voltmeter to make sure the humidistat has power (most likely 24 volts).
NOTE: Depending on the installation, the humidistat may only have power when the furnace is running.
Check for power at the solenoid
With the humidistat open, and the furnace running. Set the humidistat to the highest setting, and check to see if you have voltage going to the solenoid.
There are a lot of scenarios that can cause a pressure switch to report an error. As an example, one that I have seen that is actually "self-healing", I have seen really cold days where there are ice crystals in the air and as the air is pulled into the air intake it can create an 'ice dam' and thus restrict the air flow enough that the pressure switch does not close. This can melt if the outside temp gets warm enough and it can also be dislodged simply by shaking the intake pipe. I have also seen birds nests and various debris block the air flow. One time I am pretty sure that the wind was strong enough and blowing in just the right direction that it was preventing the switch form staying closed. (I made this assumption because of the homeowners description of his trying to fix it and their location at the mouth of a canyon and the strong winds we had had.
You mentioned power cycling the unit. That would reset the error and allow for it to try to come on again. If it is running again it should be good the switch is open and will not run unless all is well.
As far as preventative maintenance, you have done most of what can be. I have seen a pressure switch that had water in it because one of the hoses was kinked. But there is not much else that can be done.
Best Answer
It is indeed condensate water, high efficiency furnaces wring water all year long (more so during heating, but A/C in summer too).
What has happened is the condensate collection point (on the inside of where the external PVC drain runs) has cracked. It is a very brittle and cheap plastic, just saw this on another customers furnace. It acts like a little trap, holding water until it overflows into the PVC to drain.
Possibly the PVC got kicked and the force made its way back to the box (good luck getting a show of hands for this one)..
In any case, your furnace vendor will likely have one in stock.