diff -U 5 dovecot-2.2.33.1/pigeonhole/src/managesieve-login/client-authenticate.c dovecot-2.2.33.1~mwe/pigeonhole/src/managesieve-login/client-authenticate.c
--- dovecot-2.2.33.1/pigeonhole/src/managesieve-login/client-authenticate.c	2017-11-01 15:06:28.000000000 +0100
+++ dovecot-2.2.33.1~mwe/pigeonhole/src/managesieve-login/client-authenticate.c	2017-11-01 14:55:43.869493098 +0100
@@ -306,12 +306,20 @@
 		return 1;
 	}
 
 	if ( ret == 0 ) return 0;
 
-	init_response = ( client->auth_response == NULL ? NULL :
-		t_strdup(str_c(client->auth_response)) );
+	
+	if ( client->auth_response == NULL ) {
+		init_response = NULL;
+	} else if (( strncasecmp(client->auth_mech_name, "EXTERNAL", 8) == 0 ) && ( str_len( client->auth_response ) == 0 )) {
+		/* MWE: hack/workaround to pass empty response */
+		init_response = t_strdup("=");
+	} else {
+		init_response = t_strdup(str_c(client->auth_response));
+	}
+
 	msieve_client->auth_mech_name_parsed = FALSE;
 	if ( (ret=client_auth_begin
 		(client, t_strdup(client->auth_mech_name), init_response)) < 0 )
 		return ret;