(git-annex)hopa:~/datalad/neurovault/snapshots[master]
$> git annex whereis 1003/13873.nii.gz
whereis 1003/13873.nii.gz git-annex: Cannot run git-annex-remote-datalad-archives -- It is not installed in PATH (/usr/lib/git-annex.linux/bin:/usr/lib/git-core:/home/yoh/gocode/bin:/home/yoh/gocode/bin:/home/yoh/bin:/home/yoh/.local/bin:/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games:/snap/bin:/sbin:/usr/sbin:/usr/local/sbin:/usr/lib/git-annex.linux/extra)
$> source ~datalad/datalad-master/venvs/dev/bin/activate
$> git annex whereis 1003/13873.nii.gz
whereis 1003/13873.nii.gz (3 copies)
123c73e5-a8dc-4cff-8ffc-679c7ea67f94 -- yoh@smaug:/mnt/datasets/datalad/crawl/neurovault
48c1556f-6241-45de-9497-338d437fcb62 -- yoh@falkor:/srv/datasets.datalad.org/www/neurovault/snapshots [origin]
af2785da-2538-4346-a6f6-f2f30fc3f025 -- [datalad-archives]
datalad-archives: dl+archive:SHA256E-s6460020224--710cc05117e2290e2f793271d11e26452cdc111121e09a937dbf5a34b3cc0107.tar/neurovault_snapshot/1003/13873.nii.gz#size=23262
$> git annex version
git-annex version: 6.20180807+git230-gaa291acfe-1~ndall+1
IMHO output should be like (since all information is actually available)
$> git annex whereis 1003/13873.nii.gz
git-annex: ERROR - Cannot run git-annex-remote-datalad-archives
whereis 1003/13873.nii.gz (3 copies)
123c73e5-a8dc-4cff-8ffc-679c7ea67f94 -- yoh@smaug:/mnt/datasets/datalad/crawl/neurovault
48c1556f-6241-45de-9497-338d437fcb62 -- yoh@falkor:/srv/datasets.datalad.org/www/neurovault/snapshots [origin]
af2785da-2538-4346-a6f6-f2f30fc3f025 -- datalad-archives {ERROR}
datalad-archives: dl+archive:SHA256E-s6460020224--710cc05117e2290e2f793271d11e26452cdc111121e09a937dbf5a34b3cc0107.tar/neurovault_snapshot/1003/13873.nii.gz#size=23262
and indeed with error code exit may be.
Hmm, but the special remote can add urls to the whereis output, indeed it is doing so in your example. So if the repository has the special remote enabled it seems it ought to need it to be in working order.
External special remote programs are only spun up on demand, so the only way to get to this crash is when it's actually being used for something, AFAICS.