diff --git a/nixbld-etc-nixos/configuration.nix b/nixbld-etc-nixos/configuration.nix
index 6f530f25..b9ebe15c 100644
--- a/nixbld-etc-nixos/configuration.nix
+++ b/nixbld-etc-nixos/configuration.nix
@@ -1305,10 +1305,12 @@ in
   services.roundcube = {
     enable = true;
     hostName = "mail.m-labs.hk";
+    # https://github.com/roundcube/roundcubemail/issues/5869
     extraConfig = ''
       $config['smtp_server'] = "tls://${config.mailserver.fqdn}";
       $config['smtp_user'] = "%u";
       $config['smtp_pass'] = "%p";
+      $config['session_storage'] = "php";
     '';
   };