If I try when logged in as the admin user it works fine, but when I'm logged in as a non-admin user that is in the Project Manager group I get the following error:

ValidateError

Field(s) `project_escalation_id` failed against a constraint: Error! You cannot assign escalation to the same project!

Error details:

AccessError

The requested operation cannot be completed due to security restrictions. Please contact your system administrator.

(Document type: project.project, Operation: read)

I have tried setting the Project Escalation field to a different project or leaving it blank and I still get the same error.