Windows – Problems opening Excel attachments from Outlook

I'm seeing issues with Excel attachments in Outlook 2013; Trying to preview in Outlook gives error:

The file cannot be previewed because of an error with the following previewer:

Microsoft Excel previewer

To open this file in its own program, double-click it.

And when following that advice, I get error:

Microsoft Excel cannot open or save any more documents because there is not enough available memory or disk space.

• To make more memory available, close workbooks or programs you no longer need.

• To free disk space, delete files you no longer need from the disk you are saving to.

Task manager reports 4.9Gb free RAM (although its only the x86 version of Office); Explorer reports 55.5Gb free disk space; Perfmon reports Excel as only using 45Mb (private bytes), and the Excel file (generated by Reporting Services 2008) is only 56Kb, with , so I have reason to believe the error message is erroneous.

I've already tried a "repair" of Office, with no effect, and have also confirmed the program associations at Control Panel \ All Control Panel Items \ Default Programs \ Set Default Programs \ Set Program Associations

What might be causing this, and short of a complete rebuild of the PC, is there anything I've missed that I can try to fix the issue with?

EDIT: Doing some further diagnosis, I've taken a "working" Excel document, emailed it to myself, and saved it into the same folder (with a subtly different name). The two files are binary identical, however the one that has been through Outlook is "Blocked" – unblocking it through the properties page makes it work.

Now, this makes me suspect that "some security setting" has been changed, but where would I change this setting (that I don't know what it might be called)?