glibc-2.3.2/nis/nss_compat/compat-grp.c: static enum nss_status internal_getgrnam_r (const char *name, struct group *result, ent_t *ent, char *buffer, size_t buflen, int *errnop).
(This implies that, as a temporary workaround, removing `compat' from /etc/nsswitch.conf solves the problem).