These are not relationship problems. They are problems with external data source references.
It sounds like either the name of some of your files have been changed or their location--possibly their location relative to another file in your system, has changed. Even a change in the name of a folder containing one or more of your files might be responsible.
Open Manage | External Data Sources and use the entries there to make sure that each External data Source reference is valid. You may want to delete all the text inside eachof these references and use Add file to find and connect to the correct file.
Since you have multiple files, you'll need to do this in each file where you get this error.
Good luck. The external data source references in these older files are often a real mess.