These 'unfinished' should be in moodledata/temp/backup/ and in a 'contenthash' looking folder. IF there is a backup.mbz file there look at it's size. The last action is for Moodle to *copy* the backup.mbz file in there to destination as you have set up in backup preferences. What is your setting?
Many servers fail at the last step ... *copy* ... rather that 'move' ... and for a little time there one has increased space used until the copy has completed ... one has the temp folder and all the xmls + directories related + the backup.mbz file itself located there - so about 2X's the space that one course is using.
IF you left that at default backup files are going to be store in the sea of files ... moodledata/filedir/.
Recommend manually creating a web service accessible directory outside of moodledata ... like /home/mbackups/ and pointing there so you can see them easily, see their size and number of backups.
If you ever have need of more space on server, easy to remove those files ... otherwise, painful to free up space via moodledata/filedir as that involves DB queries to contenthash of .mbz flies, manually removing those contenthash named files, plus removing rows in mdl_files table that referenced them.
IF the report is showing the next backup attempt has already past then confused table.
Had a server that went nuts like that and the only way I can get it back on track was to truncate all tables in DB related to autobackups - especially the one that kept the schedule.