Klaus forwarded this to vdr-portal.de. I think vdr 2.1.8 will include a patch to add
ARGSDIR = $(VIDEODIR)/conf.d
to the ONEDIR case. Because vdr reads all *.conf files from ARGSDIR, you cannot use VIDEODIR because of setup.conf, channels.conf etc.
And I should have documented this feature in more detail (and I will do), as time permits next week.
$(VIDEODIR)/conf.d works fine. That's a little cleaner anyways so ONEDIR still works as intended, yet conf.d specific files aren't mixed together with all other *.conf.
Thanks!