Thanks for sending the project. This was a bug during import where you have a mesh keyed in the project your are importing and a region attachment in the source project. Instead of crashing, it should report warnings on import, since deform keys can't be applied to a region attachment. This is fixed in 3.6.12-beta. Cheers! 🙂:beer:
I'm not sure what error message you saw. If you post it (or a screenshot) we can take a look.