I built the code and found in sdlmain.cpp a block of code that makes DosBox behaving crazily about keyboard events.

1if (getenv("SDL_VIDEODRIVER")==NULL) {
2			if (SDL_VideoDriverName(sdl_drv_name,128)!=NULL) {
3				sdl.using_windib=false;
4				if (strcmp(sdl_drv_name,"directx")!=0) {
5					SDL_QuitSubSystem(SDL_INIT_VIDEO);
6					putenv("SDL_VIDEODRIVER=directx");
7					if (SDL_InitSubSystem(SDL_INIT_VIDEO)<0) {
8						putenv("SDL_VIDEODRIVER=windib");
9						if (SDL_InitSubSystem(SDL_INIT_VIDEO)<0) E_Exit("Can't init SDL Video %s",SDL_GetError());
10						sdl.using_windib=true;
11					}
12				}
13			}

If I remove this block, I get everything working. That's a first clue.

Try setting SDL_Videodriver=windib variable before starting DOSBox.

That will probably fix you're keyboard issue.

Removing the above code probably does the same thing. (DOSBox tries to force DirectX whereas SDL default is to use WINDIB)

Indeed, setting SDL_VIDEODRIVER=windib before running DosBox works. It will be a good workaround until I find the root cause (SDL ? x64 drivers ?).