Had this issues in .805 (https://forums.elementalgame.com/386712) as well. Alt-tabbed out of full screen mode and after approximately 5 minutes I tried to tab back in and I load a black screen with the error message stating "DX Error: Invalid call" and then the game promptly kills itself and goes back to my desktop. No memory dump was generated.

The error is indicated in the debug.err:

DebugMessage:  DX Error: Invalid call
Debug Message:  ERROR: Failed to set ambient color to -1 in ::SetAmbientColor
DebugMessage:  Error flipping back buffer to primary buffer.
DebugMessage:  Invalid call
DebugMessage:  Main Shell terminating normally

Dxdiag and debug.err are located here: http://jerome.agadak.net/elemental/Beta3A%5B.806%5D/dxerror/