A FMS hosted file is open in Pro, and has a reference to a data source on the client (using FM 16 ability to define a data source with a variable).
The local file has itself a relationship to the hosted database.
If for some reason the connection to the hosted database is lost, the local file remains open. But then you can't reopen the hosted file before closing the local file. There is no error message. Trying to open the hosted file (using launch center, recent files…) has just no effet.
Affects both Mac and Windows versions.