It is probably a bug that happens when the parameters are passed to ffmpeg renderer because it expects a dot formatted number and not a colon formated number due to locale settings.
Meanwhile, can you please run $LANG=C synfigstudio to use correct decimal point and test it again?
Also it would be so helpful if you open a bug report here: sourceforge.net/tracker/?group_i … tid=757416
Thanks!
-G