ERROR IS: CICONotCheckedOut (Project Server 2010)

our project server runs synchronization with AD every night and we find that some resources are missing overnight at the end of the synchronization and we've seen the error pretty much like this

The error is: CICONotCheckedOut. Details:. Standard Information: PSI Input Point: Project User: DOMAIN USER Correlation Id: 93c09e60-fdf9-4b0d-b48f-67251d8bd3ce PWA Site URL: https://xxxxxxxxxx.com/pwa SSP Provider Name: PSError Service of the Project Web Application Service: CICONotCheckedOut (102)

Does anyone know what kind of mistake?