I had the same issue.
My problem seems to be such that everytime i run a cron manually on the newsletter (by visiting the cron.php), it generates a series of such errors.
It is a simple error message but in MBs (some as big an hundreds of MBs) and it autogenerate till GBs of space is taken up and i got a email notification on space constrain.
I just screwed up the service provider for making me logging in every few minutes to hit the delete button until the cron finishes.
Now i’m on alert whenever i use the cron job for newsletter, always going back to Cpanel to hit delete.