"MPEG-1/2 does not support 3/1 fps" when sending webcam to ffserver

I am unable to send a stream to my ffserver.

I started the server with

where ffserver.conf has not been modified. I tried sending a webcam stream with

I have however the following error:

[...]
MPEG-1/2 does not support 3/1 fps
[...]

The port 9080 is open, as seen with nmap localhost

Why do I have this error and how to send my stream?

Full output:

Full /etc/ffserver.conf (no modification from original file):