I've recently activated notifications for testing update notifications, but seems that there's still an old URL to update download (I just look on the original topic Update notification for test builds (https://forum.openmpt.org/index.php?topic=4360.0)), so shows other obsolete link inside http://update.openmpt.org/info/testing (http://update.openmpt.org/info/testing).
It's supported the https://update.openmpt.org/check/testing/$VERSION/$GUID (https://update.openmpt.org/check/testing/$VERSION/$GUID) URL?
It's a server configuration issue on our side. We will look into it and fix it.
The download URL that OpenMPT shows for updates for testing builds will now work again.