There may have been an error with "sqlf server stop" but I didn't notice it and the "sqlf server start" overwrites that log file. The error in the log file upon start is what's mentioned in the title.
This usually happens after a 4-hour load of data. My only recourse to date is to wipe out the persistent store and start the 4-hour process again. But, then 'sqlf server stop' is likely to fail again. This is a single-node system so I need persistent stores and occasionally stop/start the server.