From the way it sounds, it failed to ignite and did a reset. It succeeded on the second try although it seemed a bit longer than I'd like. That Trane board will eventually remember how long it takes to ignite and allow more time for ignition. That's assuming the ignitor is healthy. Watch during that first period to see if the ignitor is glowing. I think it does a hard shutdown if it takes more than 3 attempts. I may be wrong on how many it allows. You may need to replace it.
I can't think of any reason the humidifier would need to have any effect on the start up of heat. I realize it may interrupt the green (fan) leg of the stat on some systems to prevent backfeeding the stat and starting up the compressor but that shouldn't bother the heat mode from coming on... especially if the fan is already running. It's no different than having the fan set in manual mode at the stat.
I can't think of any reason the humidifier would need to have any effect on the start up of heat. I realize it may interrupt the green (fan) leg of the stat on some systems to prevent backfeeding the stat and starting up the compressor but that shouldn't bother the heat mode from coming on... especially if the fan is already running. It's no different than having the fan set in manual mode at the stat.