OSX bundle v.5.1.2 : export menu is disabled

[neywen]

I launch the app, load a subtitle file (pgs stream extracted from a mkv using mkvextract tracks),
I can see the first sentence displayed in the upper window,
the log shows :
"#> 1178 (02:22:25.954)
Detected 19 forced captions.
Decoding frame 1/1178 at offset 0x000005a9"

But nothing runs after that, and all items from the File menu are disabled.

Is this a bug ? Or is there something I should do and that I don't get ?
thanks.
l.