How to reproduce:
1. Open a new project
2. Create a Shader in the Project window (Create > Shader > Unlit Shader)
3. Go to Edit > Project Settings > Graphics
4. Change the Motion Vectors value from a Built-in Shader to a Custom Shader and select the previously created Unlit Shader
5. Through the Explorer navigate to the project directory
6. With a text editor program open the "UnityConnectSettings.asset" file (this .asset file can be found in the "ProjectSettings" folder)
7. Change all m_enabled values from "m_Enabled: 0" to "m_Enabled: 1"
8. Close the Editor and open the project again

Expected result: the project opens with a Console error "Loading builtin resources is forbidden when building them: Internal-Loading.shader
If it is really necessary you can add a exception case at this assert."
Actual result: Project does not load, the following error appears: "Fatal Error! Failed to load Editor resource file"

Reproducible with: 2019.4.12f1, 2020.1.9f1
Not reproducible with: 2018.4.28f1, 2020.2.0b6, 2021.1.0a1

Notes:
- Issue only happens when "UnityConnectSettings.asset" file is edited while the Editor is running
- A workaround is to delete the Library folder after this issue appears