Course backup ad hoc task pending: 6
There are 6 courses that are in the process of backing up via ad hoc task. Ad hoc task are a backlog of things moodle was not able to finish and will try again.
Those appear to be halting autobackups as the last line suggest:
Automated backup pending: 873
Unfortuantely, the report does not say which courses so you could investigate those, thus it's a sluething kinda thing.
Check moodledata/temp/backup/ directory for long directory names. Explore those to see what's inside ... looking for hints/clues as to what courses those are. You will see 0 byte log files with longnames.log - those are from successful backups. Any .log file that is over 0 bytes might provide a clue.
I see you are saving to a designated directory. Explore that directory to see if you can determine what backups are missing (your settings show keeping 3).
Got command line access? (hope so).
Use mysql command line client to make this query:
select * from mdl_backup_courses;
That will show a table with the following headings:
id | courseid | laststarttime | lastendtime | laststatus | nextstarttime
Will help determine course ID's and the laststatus column, if it has a number other than 3 ... think 3 is successful completion ... then those are ones to be backed up. Not sure what number would show for those pending ... but there are 6 of them.
Could use the following query from a bash shell script to create a text file listing:
Then insert the following
mysql -u [user] -p'[password]' -e "use [dbname];select * from mdl_backup_courses;" > autobackups.txt ;cat autobackups.txt;wc -l ./autobackups.txt
replace  items with your info.
Save file and exit nano.
To run: source getautobu
That will create a 'autobackups.txt' file which you can then use to get a better handle on which courses are the 6.
Also, you might need to force the completion of ad hoc task to get past the 6 pending.
That is dependent upon command line access so will describe that only if you have CLI and don't suffer from 'command line avoidance syndrome'.