This includes projects created with 4.18 or 4.19 as well as projects upgraded from 4.17 and earlier (the project upgrade system is broken as well, see https://answers.unrealengine.com/questions/718575/, but this is a separate issue).
Looking at the source code, the text No is implied. is only printed by GetHeadlessMessageBoxMessage() which itself is only called if SDL would fail to initialize:
Launching Unreal Engine from the shell, this will then present you with a prompt to answer Y or N. Answering Y will compile the C++ project, open the editor and everything will work from there.
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While you seem to have worked the problem around, could you please try breaking into X11_ShowMessageBox() and seeing why it fails? There must be something specific to your X server that SDL2 doesn’t handle. What are you running - X.Org, XWayland, anything else?
I’m running vanilla X11 R7.7 (X.Org server 1.19.5) with KDE Plasma desktop 5.11.5. I’ll try to debug into the SDL code to see which of the calls to the windowing system fails.
in SDL_x11messagebox.c, there’s a function X11_ShowMessageBox(), which for some reason calls fork and then tries to read from the piped process (line #858). The call to read() from the pipe is what fails.