Right now if subtitlecomposer is installed with default use-flags and options (also with the mpv use-flag enabled), subtitlecomposer is unable to import any videos. The error is "Your GStreamer installation is missing a plug-in". There is no indication what plug-in is missing, but this makes the application basically useless. If switching the video player to "MPlayer" (but set the actual binary to mpv as MPlayer is not actually installed), it just reports an error has occured.
Please test with 0.6.6.