We are also experiencing the same behaviour. We call /calendarView/delta and page over the results using the "@odata.nextLink". Many pages work fine, but eventually, we encounter the same 503 error as described above. Contrary to the suggestion that this is just some sort of temporary service outage, it never works regardless of the number of retries.
We can re-issue the same delta request from the start and those will work and we can page over the results again, and they always work until the same page where we get the 503 error.
It's also worth noting that we can also reproduce the problem with the /events/delta api (which is only available on the beta endpoint currently).
We have a case open with Microsoft support as well. Hopefully they will be able to provide some answers.