I want to take snapshots periodically of a RTMP live video stream. I can see the rtmp video stream using VLC. This is the rtmp url:
rtmp://antena3fms35livefs.fpl
I just tried your command and it worked fine for me. Maybe it is something about your FFMPEG installation? I am using version 2.4 on a Mac (tessus build).
I know other/older versions used "librtmp" for rtmp connections, which required some extra options behind the stream URL. See ffmpeg docs here: ffmpeg documentation on librtmp
And librtmp docs here: librtmp documentation
For an unprotected live stream, you may want to try quoting the stream URL and appending " live=1" within the quotes:
ffmpeg -i "rtmp://antena3fms35livefs.fplive.net/antena3fms35live-live/stream-antena3_1 live=1" -f image2 -vf fps=fps=1 out%d.png