This error message indicates that there is some corruption in your project metadata. This may be the first time that you've encountered a fatal error, but over time this sort of thing is bound to accumulate warnings, errors, and inconsistencies. If you're unlucky, you may one day find that your project has suffered a serious data loss -- in extreme cases, you could end up corrupting every scene and prefab in the project.
To avoid that, there are some best practices for sharing Unity projects between multiple computers:
- When possible, use only one consistent version of Unity
- Switch to
Visible Meta Files in Edit → Project Settings → Editor → Version Control Mode
- Switch to
Force Text in Edit → Project Settings → Editor → Asset Serialization Mode
- Avoid copying the
Library and Temp folders between computers; instead, let each computer rebuild those folders using data found in the meta files
Addendum to item 4: I often copy/share only the Assets and ProjectSettings folders from any given project. As long as you're using meta files, those two are usually the only folders that you need for Unity to recognize and rebuild a project folder.
Skipping any of these steps can cause you to encounter some very esoteric corner cases in Unity's asset serialization systems.
Ideally you can implement this before starting serious work on a project. If that ship has already sailed, I would recommend applying those steps to the most stable copy of your project, which you can then share to other computers.
