A: This is not uncommon, as the moisture in the box sometimes gets “under” the glue. It’s happened to me a few times. I even used Gorilla Glue on one of the back-plates for a magnetized unit and it hasn’t affected the box at all. Just note that if glue is your only option, be sure it’s an odorless glue.
First, I would suggest that you lightly sand the area on which you plan to affix the humidifier to get some fresh dry wood. If the unit has a metal back-plate, you can use contact cement like Gorilla Glue or Super Glue. If it’s plastic, try Velcro, which is easier to replace in case the problem repeats itself down the road. Velcro can be used on a metal back-plate, too, but again, make sure you have fresh wood, so the glue adheres properly.
Finally, NEVER glue the humidifier, itself, to the humidor.