Your df doesn't show /tmp so presumably it is part of the root (/) filesystem. The root filesystem is only 74% utilized at the time you ran df -h so it doesn't seem likely your issue was space. However it isn't impossible - some processes will create large files in /tmp and fill up the filesystem then die - when they die they delete the files they were trying to create. In fact your original message shows such a deletion occurring:
Output file "/tmp/file1IJwvk" removed
I notice for your du output you don't list /tmp. Have you verified /tmp exists (run ls -ld /tmp)? If you run "df -h /tmp" does it in fact confirm it is part of the root filesystem (that is does it give the same output as "df -h /")?
You should also examine /var/log/messages to see if it has any errors (especially any indicating a full filesystem).