Andy,
We've seen this error in many scenarios. Obviously, it CAN be related to hard disk space, but it could also have other causes not necessarely related to disk space.
What version are you on? If you're not on the latest version, I would recommend that you upgrade, particularly if you are still on 7.2.4 or lower. There were memory issues where if it would run for a long time, it would eventually fail to produce jobs properly and throw all kind of errors(or no error at all sometimes and just hangs), forcing us to restart services. I would recommend upgrading to 7.4.0.
Also, try rebooting your machine. We've seen cases where a simple reboot is all it took to fix the problem. If the machine has been running for a long time without reboot, it's possible that many things that have been installed are starting to interfere with each others, and a reboot usually put things back into place.
Regards,
Raphaël Lalonde Lefebvre