Thanks for the bug report.
This is definitely not a limitation on the provider side.
This is some kind of bug in RWT and / or Unity Editor, which is caused by resetting the state of a variable, under some conditions.
I have corrected this place. The patch will be released next week.
But it seems to me that this error happened after an error somewhere else.

NullReferenceException: Object reference not set to an instance of an object
InfinityCode.RealWorldTerrain.Net.RealWorldTerrainDownloadItemUnityWebRequest.Dispose () (at Assets/Infinity Code/Real World Terrain/Scripts/Editor/Net/RealWorldTerrainDownloadItemUnityWebRequest.cs:54)
InfinityCode.RealWorldTerrain.Net.RealWorldTerrainDownloadManager.Dispose () (at Assets/Infinity Code/Real World Terrain/Scripts/Editor/Net/RealWorldTerrainDownloadManager.cs:76)
InfinityCode.RealWorldTerrain.Windows.RealWorldTerrainWindow.Dispose () (at Assets/Infinity Code/Real World Terrain/Scripts/Editor/Windows/RealWorldTerrainWindow.cs:176)
InfinityCode.RealWorldTerrain.Windows.RealWorldTerrainWindow.CancelCapture () (at Assets/Infinity Code/Real World Terrain/Scripts/Editor/Windows/RealWorldTerrainWindow.cs:54)
InfinityCode.RealWorldTerrain.Windows.RealWorldTerrainWindow.Update () (at Assets/Infinity Code/Real World Terrain/Scripts/Editor/Windows/RealWorldTerrainWindow.cs:395)
UnityEditor.HostView.SendUpdate () (at <756e9f00f78640ec93f83e08257daaec>:0)
UnityEditor.EditorApplication.Internal_CallUpdateFunctions () (at <756e9f00f78640ec93f83e08257daaec>:0)

Some time even with different data providers. I'm guesing that there is some service limitations from those data providers but didn't find any info about it.
Do you know if there are limitation of data amount of any kind or any change in the services conditions? I never had this problem before. I have been trying SRTM, SRTM30 and BingMaps
Thanks,
Joan