If you've checked that its not your disk quota then that takes out half the error. Leaving just the "unknown error" part which isn't always helpful.
Are you trying to upload via FTP? And if so are there any large files you are trying to upload?
cPanel seem to suggest that mod_security rules or a large file (larger than the allowed amount) is the cause of the error if not the disk quota.
If you have access to WHM (I've forgotten what package you are on) you can try to increase the disk quota amount for that site to see if that makes a difference.
If nothing of the above seems to apply then I'd suggest raising a ticket, maybe theres another problem thats not covered here.
Hope that helps
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