This happens when the saved report becomes a subscription to the Subscriptions page and no longer to a graphical report, like Cost History.
The example below explains how this can happen:
From the Saved Reports page, click on number of subscribers (see Figure-1). This will direct you to the subscribers of that saved report in the Subscriptions page (see Figure-2)
Figure-1 Saved Reports: Cost History - Test
Figure-2 Subscriptions: Cost History - Test
If you click on Actions > Save in Figure-2 above, this will open Figure-3 below:
Figure-3 Actions>Save
Saving this report regardless of what name you choose will override the original saved report in Figure-1 and makes it point to the Subscriptions page URL in Figure-2 and no longer point to the actual Cost History report URL.
Delete the saved report which will automatically delete the subscription as well and recreate the saved report to resolve this. The platform is behaving as intended despite potentially being confusing.