My receiver (Topfield SRP 2410) has the nasty habit of rescaling to fill the display, ignoring the aspect ratio. So I am reencoding, adding padding (black bars) and top and bottom. In this context avconv has offered some less cryptic errors.
The one that cost me most time was this one:
[libx264 @ 0x8f29ca0] constant rate-factor is incompatible with
2pass.
[...]
Error while opening encoder for output stream #0:0 - maybe incorrect
parameters such as bit_rate, rate, width or height
libx264 interprets a missing bit-rate specification as a request for constant rate-factor encoding. Which does not work with multi pass. Setting a bitrate (avconv syntax -b:v 1200k) fixes the problem.