I see, it looks like when you mass-select the trees and set 'immovable' to 'yes', it actually blanks out the combo box. If you do it again, it works, but if you leave it blank it stays blank. Because it says neither 'yes' nor 'no, you get an error on export.
It's easy to work around - just select the combo twice and it works - but I'll try have it fixed before I go on holiday.