After reboot: Failed to add bandwidth usage

These are usual race condition errors, your node was not fast enough for that customer.

In short - your node has some access errors when this endpoint is called, you may check what is it in your logs on the time when you refresh this URL.

You may try to use a different method to request your node’s hostname and port, I do not know what is UptimeKuma uses as a default method (GET, etc.), so you may experiment here.