The bug is already reported (it’s most probably this : Unity Issue Tracker - Mesh Collider throws an error when upgrading to 2019.3.0a9 with mesh not marked as readable ). The tl;dr; is mesh colliders that are in the “default” state, are not upgraded properly (probably because the “Use Fast Midphase” was added).
It stops me from upgrading our project from 2018.4 to 2019.3, since all mesh colliders start complaining about not being readable, even though they should be in the “default” state and I need to go in every scene and every collider to reset them. (I can’t even easily script it, since AFAIK there is no way to set the cooking options to “default” via script, although I guess I could remove the mesh colliders and re-add them)
This was reported a while ago, I’m hoping it’s planned to be fixed for the release.
Also… maybe more long term, can you maybe fix the “default” state being so weirdly defined? Maybe add another checkbox that says “Default” which when ticked it disables editing of the cooking options? Regardless of the bug here, the current behavior, even if it was working properly, is very confusing.