Encoding fails if no cache-initial is used (in some cases)

mpv version and platform

git master as of 2017-09-24
combined with youtube-dl from 2017.09.15

Reproduction steps

Run:

mpv -profile enc-to-nok-n900 http://www.ardmediathek.de/tv/dings-vom-dach/dings-vom-dach-ganze-Sendung/hr-fernsehen/Video\?bcastId\=14847140\&documentId\=41161108  -o 2017-03-05.mpg

I explicitly renamed by ~/.mpv/config and encoding-profiles.conf.

This fails (see "actual behaviour" below), while running:

mpv -profile enc-to-nok-n900 http://www.ardmediathek.de/tv/dings-vom-dach/dings-vom-dach-ganze-Sendung/hr-fernsehen/Video\?bcastId\=14847140\&documentId\=41161108  -o 2017-03-05.mpg --cache-initial=2000

i.e. just adding --cache-initial=2000, makes things work.

Expected behavior

Things work.

Actual behavior

Playing: http://www.ardmediathek.de/tv/dings-vom-dach/dings-vom-dach-ganze-Sendung/hr-fernsehen/Video?bcastId=14847140&documentId=41161108
Can not open external file http://www.ardmediathek.de/subtitle/140626.
 (+) Video --vid=1 (*) (h264 960x540 25.000fps)
 (+) Audio --aid=1 --alang=deu (*) (aac 2ch 48000Hz)
Opening video filter: [lavfi graph=scale=floor(min(min(800\,dar*480)\,in_w*max(1\,sar))/2+0.5)*2:floor(min(min(800/dar\,480)\,in_h*max(1/sar\,1))/2+0.5)*2,setsar=sar=1]
VO: [lavc] 800x450 yuv420p
[encode-lavc] option --ofps not specified and fps could not be inferred, using guess of 24000/1
[encode-lavc] Opening video encoder: libx264 H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10 [libx264]
[ffmpeg] libx264: MB rate (34800000) > level limit (40500)
[encode-lavc] no audio stream succeeded - invalid codec?
[encode-lavc] vo-lavc: encoded 0 bytes
[encode-lavc] ao-lavc: encoded 0 bytes
[vo/lavc] NOTE: skipped initial video frame (probably because audio is not there yet)


Exiting... (Quit)

Log file

Failing log file:
http://sprunge.us/jheD

With cache, things work:
http://sprunge.us/XNRT

Sample files

I think the URL I used is publicly accessible for free (at least it is in Germany, not sure if they have geoblocking).

Reproduce the command from the issue with and without --cache-initial=2000, then compare the two linked logs and the encoding profile enc-to-nok-n900. Trace the cache and encoding entry points implicated by the differing behavior; done means the reported URL encodes successfully without explicitly setting --cache-initial and the regression is covered by an appropriate test.