Perhaps the error has something to do with the PNG format or the lack of support thereof.
Here is a manual quote which seems to suggest that the PNG format should not be used:
PNG images behave like TGA images, but are not recommended because they are loaded not by the engine, but by the DirectX library, and are not guaranteed to be automatically restored when the video device gets lost. This means that they can lose their content f.i. when a fullscreen application is minimized.
That quote is from an out-dated manual.
...
case none: invent_pan[i].bmap = NULL;
...
