by Ken Task.
Turn autobackups off.
Manually remove all the contents of moodledata/temp/backup/
After that, do what many Windows server admins do ... reboot the box.
Check the reports for autobackups in Moodle admin. You might have to truncate all tables moodle keeps related to autobackups to 're-gain control' .. perception of 'control', I should say.
'spirit of sharing', Ken