This type of error occurs when a project is in both CUS and USR layer.
You can go through all your projects and export project to an xpo which has both CUS and USR layer and delete the USR layer of the projects.
Or
To find out to which project this error is referring to,
Write a SQL Query in Model Database
select * from modelelement
where elementhandle = '1120873'
and then export an xpo of the project and delete the USR layer.