It is quite hit and miss.
Ive resolved by just simply restarting WE, sometimes it wont work if you dont have your default layer active (or sometimes the layer that your SCR_MapEntity resides in.
an export folder HAS to be defined , but sometimes purely changing that folder to anything outside of your main terrain folder would let you export. sometimes it will only let you export to your main folder ...
i get an error where my Log Console will tell me it s been exported okay, but it hasnt. doing some or all of the above seem to fix this
been doing this since the first few days of the tools being released and only issue i get is the TGA is horizontally flipped, i have to flip it in PS before converting to .edds
glad you got your working eventually 🙂