I once had an issue similar to this, turned out to be a stick issue more than anything else, but just in case I sequestered all my sticks into a tupperdore while I went to work on the humi. I am kinda anal about possible mold issues lol, so I'm sure I went overboard. At the time I was using a combination of foam humidification and beads as well.
I first aired out the humi completely for a couple of weeks, I then proceeded to wipe down the inside very a little (very little) isopropyl alcohol, and then I sanded every portion of the inside of the humi and the tray with a very fine grit sandpaper. If you are going to go this route, you need to be careful with how coarse the sandpaper is and how you sand. Too much pressure in one spot and you can end up with hills and valleys in the wood. I think i used somewhere around 600 grit, but I know it was a "finishing" type of sandpaper so it may have been higher than that. Do not just "go to town" with the sanding, you arn't roughing out a floor or anything here lol. I used very light pressure and took off very little....enough that you could see some fine dust, but not enough that you have to get a broom out to sweep up. I went over everything one time, and when I was satisfied, I vacuumed out the humi and the try to get all the fine particles out, and then re wiped with the isopropyl.
The last step after letting it air out for a couple more days was to re-season the humi, and this time I went with beads only. In fact I ditched my old beads (anal about it, like I said) and went with new to be sure that nothing was lingering behind in the beads. Its been fine ever since.
I will caution you again about the sanding, fine fine grit and very light but uniform pressure. Also you want to be careful around the sealing edges....take too much off there and it may not seal correctly. I was very careful there as well.