Please try the following, some are for validation only
What environment is this? Is it the default of your own Developer, Sandbox or Production Type?
1. Publish all customizations (app is closed)
2. Make sure it is in a solution
3. Export the Solution as unmanaged
4. Import it into another environment and see if that then works there
5. Are you using Modern controls or classic? and did you turn on any specific Setting, or use any Preview controls?
6. Is it just this app or others?
7. Is it published and Live for people to use? if not. And Exporting it works on another machine, preferably and in another environment, you can clean up the app by deleting it and anything it has from the Default solution, then re-importing the unmanaged solution that I hope is working.
Also is this a MAC or a Windows machine?
I am wondering if this is a coauthoring bug with someone leaving it open
Is there anything in the App.OnStart and if so what is it? what about the OnVisible?
Lastly if you need too, setup a VM that you can use to Import into, again in another environment, even if you need to setup a Developer environment
If you do import it somewhere else, make sure codeveloment is off.
