The Scheduled Backups uses a temporary folder of C:\Windows\Temp.
We've found that when your server's drive C is getting low, this causes issues. We have experienced a C drive with 21GB free being reduced to 400MB due to the Sage backup temporary files not being deleted after a few backup runs, say over a weekend. This is critically low.
If this temporary path was able to be edited, just like the final backup destination path is, then it could be e.g., D:\Sage\Temp.