by Ken Task.
Under 'any advice' ... but more like 'more exploring for reasons'
You are not alone ... others reported similar and there doesn't appear to be a single cause/solution ... so having said that ...
Exploring: what's in moodledata/temp/backup/?
Run cron.php from command line and note adhoc task.
More exploring: there is table for adhoc task ... view it.
'SoS', Ken