Password-protected dirs working but reporting non-existent errors

[TinyApps.Org]

A quick question if anyone can help:

Password protected directories are working fine,
but when a legitimate user logs in, the following
appears in the error log:

[error] 3876#0: *12 no user/password was provided for basic  
authentication

despite the fact that the username and password
*was* supplied.

A bad username/password produces the expected error:

[error] 3876#0: *12 user "whatever" was not found in "/etc/nginx/htpass"

Has anyone else seen this or have any idea why such
an error would be triggered?