I have had this issue with all versions of FXSound for pretty much a year.
My setup have 2 monitors, one of them is an actual TV. If everything is turned on, FxS properly identifies that I have my normal speakers (P10) and the “LG TV” outputs and I can choose freely which device to output. Fine.
However when I turn off the TV, sometimes FxS resets the list of possible devices to nothing. Sometimes when the screensaver starts up and the TV enters standby, this will happen by itself. Other times, when I have only my speaker on and I turn the TV on, upon refreshing the output list it will not display the TV, or go blank again.
For some time I also had a bluetooth speaker and same thing would happen most times when I turned the speaker off. Instead of returning to the PC speakers, the list would refresh to blank with no output options.
Basically … when the list of output devices changes, specifically the device in use is disconnected, 90% of the times FxS will bug. Only fix is to manually close it and restart it.
Not a killer, but very annoying bug. As a developer I would offer this piece of quick fix: add a “refresh list” button to the output list.