Issues Solved, posting for others.
Would host a lobby or game. After a few minutes, the screen would flash white. Game would then crash if I tried to alt-tab out. Seemed to be caused by an issue with DirectX and the game losing access to the graphics card. The error in the log is:
warning: c:\work\rts\main\code\src\libs\gpggal\DeviceD3D9.cpp(866) An invalid parameter was passed to the returning function
I searched the web and tried various fixes, non of which worked:
-Reinstall Supreme Commander
-Reinstall Graphics Drivers
-Set game to Windows 7 Compatibility Mode
-Under Compatibility, Disable fullscreen optimizations
(Note, some of these seemed to have worked for other people)
I use a Lenovo Laptop with a QHD screen. The game at this resolution makes the UI very small and difficult to read, so I set my in-game resolution to lower. This wasn't a problem until the last few days, so don't know if it is somehow related to the new FAF Client, where if I Alt-Tabbed any time after the screen had flashed white, the game would crash with the above error. If trying to host a game immediately after a crash, the game would crash without even loading the lobby from the same error. Restarting the computer was the only way to host again.
Setting the game to the native resolution of the screen, the screen never flashes white and will not crash when alt-tabbing.