Rendering reaches 2%, then jumps to 100% ... output only a couple of seconds long

Was able to render files fine (running Kdenlive on Ubuntu 12.04.)

Then did system update - uh oh, now rendering will always reach 2%, then jumps immediately to 100%. I do get output, but it is only a few seconds long.

Have reinstalled libavcodec and friends (upgraded to -extra-## package), no luck.

Where next?

Many thanks