[Zubersoft]

MobileSheets needs to create the backup file in the internal storage of the tablet before uploading it to OneDrive. Can you verify that the internal storage shows that you have at least 600 MB free? I'm just not sure if when you mention the 2.5 GB, you are including a removable SD card, or if your tablet has multiple partitions for application data. Alternatively, just backup first to a folder on your device and upload that backup to OneDrive separately.

Mike