MAST doesn't exactly import meshes, unless you're referring to the tool that creates prefabs out of your models. There are some known bugs with that tool, but you can create your own prefabs without it. Also, if you started trying to create prefabs and received an error, you need to delete the materials folder it created before trying again.
If you're not using MAST's prefab creator and still getting errors, then I recommended deleting all files in FSP\MAST and reinstall MAST from scratch. I made some naming changes to classes over different versions and old with new files together, you can get errors.
If you need further help, you can reach out to me on Discord:. https://discord.gg/eMKJ5U5
Keith at FSP