With both files open and hosted, use a full access password to connect to each of these two files. (You can ignore these error messages for now.)
In each file, open Manage | External Data sources and check the references from each of these files to the other. It sounds like you have issues in these external data source references in both files. You can Delete the entire reference text and then use the Add File button to add a brand new file reference to the hosted file. You can use the remote option to connect to it just like you would use Open Remote to access the file when you first launch FileMaker.