A backup fails with the error "There is not enough space on the disk".
This is seen when there is not enough space on the relevant drive to create a VSS snapshot in order to perform a backup.
Restarting the service will not solve the problem. You will need to ensure that the relevant drive has sufficient capacity before retrying.
You can also lower the retention of indexes to 7 days, which will clear up some space on the next backup. To do this, open your ESE agent and go to Options > Advanced > Working Folder. Select the option accessed within the last and ensure the number is set to 7. Click OK.
Alternatively, you can change the working folder to a drive that has more space. To do this, open your ESE agent and go to Options > Advanced > Working Folder, then click Change and select a new location.