"Unable to instantiate prefab. Prefab may be broken"

hey Guys! the solution for that is (Right click on the prefab and then hit Reimport)

So, 2018.2 made some changes to prefabs that break things when you try to downgrade. I've found two solutions, but I've only tried the first.

No matter what you do, make sure you've backed up your project first.

Solutions

1. Break the prefabs in the 2018.2+, in your scene. SAVE your scene. Then open that scene in the older Unity version (either by exporting and importing, or by copying your project or files). The prefabs that are in the scene, that you broke the connection should still work. You can break a prefab in the "GameObject" menu after selecting a prefab. There's some more info here https://answers.unity.com/questions/1502308/missing-prefabs-after-downgrading-from-unity-20182.html

2. You can try modifying the files themselves from the console as outline here https://www.reddit.com/r/Unity3D/comments/8gl8og/downgrading_from_unity_201820b2_to_unity_201810f2/

Be aware that other things may break also, when downgrading(which is my problem, right now).

This is most likely the same issue as in this question over here. The problem is that Unity seems to have changed it's prefab serialization format and didn't make it backward nor forward compatible. It's mainly a rename of some fields. If you use text serialization you could rename those fields yourself. However i strongly recommend you create a backup of your project.

Also if you can verify that this is the problem, please file a bug report with as many details possible. So create a new standalone project where you copy that offending prefab to and attach that project to the bugreport. The more people verifying the issue, the faster this may be fixed.

As a general advice: If you are in the middle of a production, do not upgrage Unity. Even Unity doesn't recommend it.

If you have make a backup you can download unity hub and run your project with Unity 2017.3.