Occasional 503 Error when using webhook

Has anyone encountered an issue when using WebHooks through Nabu Casa ?

I have a Fingbox network security device (for which I believe there is no native HA integration). This unit can provide presence info which is useful to me in HA.
I’ve setup a IFTTT applets to form a bridge, one informs when a person arrives, the other when a person leaves. 98% of the time it works fine but occasionally IFTTT fails with the following error:

Your server returned a 503. Unable to make web request to https://hooks.nabu.casa

Research suggests it is the Nabu Casa server not responding to the request ??

Has anyone encountered errors like this ?
I assume I’m right in looking in the right direction. i.e it’s Nabu Casa failing to respond rather than IFTTT failing to send.

Any help will be appreciated