av_interleaved_write_frame() unknown error when streaming WebM - ffmpeg

Question

I'm following a guide to live WebM streaming through FFMpeg / FFServer and running into an interesting error. I have tried using a DirectShow webcam source, and also an existing WebM (finite length) video using -vcodec copy. Initially, both will manage to connect to the FFServer (I can see the POST 200 OKs to /feed1.ffm), and maybe even send a frame or two, but then FFMpeg crashes with av_interleaved_write_frame(): Unknown error. (Meanwhile, FFServer appears to be fine.)
This appears to be an unusual variant of the error - normally it's more common to get, say, av_interleaved_write_frame(): I/O error (which indicates file corruption). Has anyone seen this error, and better yet, can anyone tell me how to fix it?
FFMpeg commands
ffmpeg -re -i univac.webm -vcodec copy -acodec copy -f webm http://[my server]/feed1.ffm
ffmpeg -f dshow -i video="[my dshow source]" -f webm http://[my server]/feed1.ffm
FFserver command
ffserver -f ffserver.conf
ffserver.conf
This is only a slight variation in the one provided in the aforementioned guide.
Port 8080
BindAddress 0.0.0.0
MaxHTTPConnections 2000
MaxClients 5
# MaxBandwidth 10000
CustomLog -
NoDaemon
<Feed feed1.ffm>
File ./feed1.ffm
FileMaxSize 1G
ACL allow [IP of the machine with ffmpeg]
</Feed>
<Stream test.webm>
Feed feed1.ffm
Format webm
# Audio settings
AudioCodec vorbis
AudioBitRate 64
# Video settings
VideoCodec libvpx
VideoSize 640x480
VideoFrameRate 30
AVOptionVideo flags +global_header
AVOptionVideo cpu-used 0
AVOptionVideo qmin 10
AVOptionVideo qmax 42
AVOptionVideo quality good
AVOptionAudio flags +global_header
VideoBitRate 400
# Streaming settings
PreRoll 15
StartSendOnKey
</Stream>
FFserver logs
avserver version 0.8.6-6:0.8.6-1ubuntu2, Copyright (c) 2000-2013 the Libav developers
built on Mar 30 2013 with gcc 4.7.2
AVserver started
[current time] - [GET] "/feed1.ffm HTTP/1.1" 200 4149
[current time] - [POST] "/feed1.ffm HTTP/1.1" 200 4096

Answer 1

This is probably caused by using different versions of ffmpeg and ffserver.
Try to use the same version. They should work without a problem.
In addition, use only libav or ffmpeg, because they are probably not quite compatible with each other.

Answer 2

The connection was establish by tcp and after I get the error on the client 'av_interleaved_write_frame(): Unknown error'. And I get 'Connection timed out' on the server.
For me, I found that I had an other process listened on the same port that ffmpeg was configured to used on the client.
to check used ports command:
(windows) netstat -a -b
(ubuntu) netstat -a -p
I used a custom ffmpeg inside a folder. The command used 'ffmpeg' used the wrong ffmpeg. Then I change it to './ffmpeg'.