the current toolkit is incompatible with Blender 2.90. The next version will address this, but in the meantime, I'd recommend you switch to 2.83.6.
Check the emissive color modifier for the material. If you're working on dynamic mesh-lighting for aircraft, make sure to set it to black [0,0,], if you're working on scenery, set it to white [1,1,1]. It works fine from my experience so far.
If you're having trouble with this since the last update, I'd recommend you re-initialize the material by changing the material mode to something else and back. Your material's settings should be maintained, but this ensures that the color values are hooked up correctly. There was a but in a previous version that set the emissive to red under some circumstances.
We have been discussing this issue internally and it'll be addressed with the next update. In the current version, the GUID is stored in the scene, so whenever you export, the same scene's GUID is added to the model's xml file. In the next version the exporter will check if the xml file already exists and has a GUID . If so, it'll recycle that GUID , if it's a new xml file, a new GUID will be generated.
These type of issues only pop up as we're using the tool, so if you have similar suggestions how to improve, let's hear it!