On Wed, Feb 27, 2019 at 06:02:06PM +0100, Marco van Hulten wrote:
> The full traceback has been saved in /tmp/guix-build-mpd-mpc-0.31.drv-0/sphinx-err-1gyuv1e6.log, if you want to report the issue to the developers.

[...]

> Where to find the *.log file (which is probably on LSB distributions on the named place)?

It should be there, if you ran the command with --keep-failed.