This is a fringe case. It appears if, in the past, the API endpoint has been updated in RemoteSpark. When the software updates, the endpoint, if it has been edited in the past, does not update.
The are two possible solutions:
Uninstall the RemoteSpark software from your device and install a fresh version of it from the Microsoft Store (or sideload if that method is not available). This will ensure that your endpoint is updated to the most recent version and subsequently be kept up to date as you keep your software up to date.
You can manually change your endpoint to the latest version within the RemoteSpark application.
On the login page of RemoteSpark, click the gears icon at the bottom right of the modal to open your settings.
Under Select Server click Edit Item.
Replace the URL with the latest Traffic Manager Endpoint found here. Save your settings once complete.