Sadly this is what happens when the mailbox corrupts.
You could try getting the jobs via the software (Fiery Remote Scan) instead of the webpage, but it is probably a lost cause.
Assuming nothing critical is on there, just clear out the mailbox of all scans (CWS5 > Server > Clear Server > Scan).
Reboot and try another scan.
If it doesn't begin to work on the new jobs you would need to create another mailbox to use or reload the Fiery.
I am able to scan on the C75 and it creates the file. When I connect through the C75's IP address, I can see the document in the Docs tab. However, when I click on the filename, it now says Preview Not Available and when I try to download the file, a pop up window says "Not able to save document." This is a recent development, as we have been successfully downloading scans on the C75 for 3+ years. Any idea what could be causing the problem?