You should carefully check your code and all custom addons that you are loading. Make sure that the same model is not defined more than once with the same name.

If you find multiple definitions of the same model, rename them to avoid conflicts. Right now, the issue is likely caused by having two or more models with the same name. As a result, Odoo is unable to find the field — because the last loaded model might be missing it the field. 

My issue was that the previous Python process was not terminated correctly. Somehow, I had two processes listening on the same port (lsof -i :8069), and my frontend was served by the old one. I terminated the process (kill <process id>) and everything works now