Up to VASSAL 3.4.3 there would be text output in the Error Log in the Module Manager (Help > Show Error Log). In VASSAL 3.4.4 there is no output, the window is simply empty white.
VASSAL 3.4.4: attachment
3.4.5 (out “momentarily”) will fix this. Errorlog is still being created, but the window is not finding it properly.