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 FAF
-Reinstall Graphics Drivers
-Reinstall DirectX
-Update BIOS
-Reinstall Java
-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.
Solution:
Setting the game to the native resolution of the screen, the screen never flashes white and will not crash when alt-tabbing.