Do not lookup from errToCode map when error is of type slice.

Closes #631.

internal/api/helpers/errors.go

 package helpers
 
 import (
+	"reflect"
+
 	"github.com/pkg/errors"
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/codes"
@@ -45,6 +47,13 @@ func ErrToRPCError(err error) error {
 		return cause
 	}
 
+	// Some error values (error returned by Otto) can not be hashed:
+	// https://github.com/brocaar/chirpstack-application-server/issues/631
+	k := reflect.TypeOf(cause).Kind()
+	if k == reflect.Slice {
+		return grpc.Errorf(codes.Unknown, cause.Error())
+	}
+
 	code, ok := errToCode[cause]
 	if !ok {
 		code = codes.Unknown