its so cool that i know the solution to this problem as i use clipbucket my self and i have faced this problem
there are two problems one ur ffmpeg is too new and now compatible with clipbucket
0.9.1 is the version ie compatible and u need to Turn on VF in convertion settings
and i guess if u also face this problem with the images then your php is not compiled with gd2 library so u need to install that
I hope this helps