Before SERVER-7160 was fixed, doing a restore on a server with auth enabled with the correct username and password for a single DB other than the admin db will trigger this assertion. This is caused by the restore code trying to print out the "need to login" error message but because of this bug in the logging code, this assertion came out instead.

Offending code @ restore.cpp, createIndex():

if ( ! ( err["err"].isNull() ) ) {
    if (err["err"].String() == "norepl" && _w > 1) {

condition should probably be

if (err.hasField("err")) {
 ...
}
else if (err.hasField("errmsg")) {

and

if (err["err"].str() == "norepl" ...