Furnace entering lockout mode - air flow problem?
I'd have to say to get an HVAC technician in for a complete cleaning, overall inspection, adjustment and, of course, solving the problem...keep using the better filters, they're worth it to you and to the system.

Yes, it could very well be an air-flow problem and the filter removal may have partially addressed the problem. However, if you have cooling on top, then the filter removal, possibly, revealed the problem's deeper and you have a dirty A-coil. This, is what the air always blows through regardless of heating or cooling.

And yes, it could be your humidifier if a majority of air flows through it with your setup...uncommon for what I've seen. However, your blower fan or even burners could be clogged-up or you may have a circuit board, transformer, sequencer, limiter or even igniter that's failing.

Sorry, HVAC is just a bit too complicated anymore and even the codes thrown by them are commonly just a symptom and not the cause. If the unit's never been serviced since 2009, then it's definitely due and may have been wasting money for a couple of years.