test@test:~$ cvlc screen:// --screen-fps=25 --input-slave=alsa://hw:Loopback,1,0 :sout='#transcode{vcodec=mp2v,vb=8000,....
VLC media player 4.0.0-dev Otto Chriek (revision 4.0.0~rc1~~git20180124+r73771+120~ubuntu16.04.1)
[000000000245a090] dummy interface: using the dummy interface module...
[mpeg2video encoder @ 0x7f8b34029300] [Eval @ 0x7f8b5c737e40] Invalid chars '.0' at the end of expression '1.0'
[mpeg2video encoder @ 0x7f8b34029300] Unable to parse option value "1.0"
[mpeg2video encoder @ 0x7f8b34029300] Error setting option qsquish to value 1.0.
[00007f8b34000f50] avcodec encoder error: cannot open mp2v video encoder
[00007f8b34000f50] main encoder error: S'ha produït un error en la transmissió/transcodificació
[00007f8b34000f50] main encoder error: No s'ha pogut obrir el codificador mp2v vídeo.
[00007f8b54007860] stream_out_transcode stream out error: cannot find video encoder (module:any fourcc:mp2v). Take a look few lines earlier to see possible reason.
[00007f8b54007860] stream_out_transcode stream out error: cannot create video chain
[00007f8b54014fe0] main decoder error: cannot create packetizer output (RV32)
[00007f8b54014fe0] main decoder error: buffer deadlock prevented
Not fail with avcodec=h264 (I have not tried others).
But the most curious is that with the C locale, preceding LC_ALL=C in commandline or exporting LC_ALL=C variable, work correctly with all codecs.
test@test:~$ LC_ALL=C cvlc screen:// --screen-fps=25 --input-slave=alsa://hw:Loopback,1,0 :sout='#transcode{vcodec=mp2v,vb=8000, .....
Not problem with vlc 2.X version.