TOPIC:

If you want multi-platform compatibility including iPads then the video files uploaded directly need to be .mp4 rather than .m4v - the latter will show as a download file link on ipads. However I don't think it's the format that is causing the export problem as I just tested an export containing an m4v which worked fine.
Ron

Thanks again Ron.
So I added a single .m4v and that broke the zip download (again by producing a 0 byte file). So I added an mp4. Same result (0 byte zip).
So removed the mp4 and then tried export just with .pdf documents (which worked prior to adding the mp4) and that is now not working.
which makes me wonder if there is some caching issue?

What browser? Try a different browser? Fully clear offline files/browser cache.
Is your installation public facing? If so make the project part of the export feed via properties and share the link.
Not a problem I can replicate so seems specific to your setup.
HTH
Ron

I've tried different browser across different OS..same result

Turned the debugging on and in the log I'm seeing

2015-10-09 12:46:05 /srv/www/vhosts/xerte.uct.ac.za/xertetoolkits/library/Xerte/Zip/Native.php67Adding models/mcq.rlm (from: /srv/www/vhosts/xerte.uct.ac.za/xertetoolkits/USER-FILES/6-01378575-Nottingham//models/mcq.rlm) to zip
2015-10-09 12:46:05 /srv/www/vhosts/xerte.uct.ac.za/xertetoolkits/library/Xerte/Zip/Native.php67Adding models/inventory.rlm (from: /srv/www/vhosts/xerte.uct.ac.za/xertetoolkits/USER-FILES/6-01378575-Nottingham//models/inventory.rlm) to zip
2015-10-09 12:46:05 /srv/www/vhosts/xerte.uct.ac.za/xertetoolkits/library/Xerte/Zip/Native.php67Adding preview.json.8 (from: /srv/www/vhosts/xerte.uct.ac.za/xertetoolkits/USER-FILES/6-01378575-Nottingham//preview.json.8) to zip
2015-10-09 12:46:13 /srv/www/vhosts/xerte.uct.ac.za/xertetoolkits/library/Xerte/Zip/Native.php98Wrote : 0 . bytes ... hopefully
2015-10-09 12:46:13 /srv/www/vhosts/xerte.uct.ac.za/xertetoolkits/modules/xerte/export.php397Zip file errors?

but in /tmp I see
-rw-r--r-- 1 wwwrun www 149449914 Oct 9 12:45 xerteExportEbunX9

(logs and /tmp zapped prior to attempted export)

That shed any light on issue?

Just to be clear you have other projects that do export ok correct?

I'm about to go offline for a while but Tom knows the export code better than anyone so hopefully he'll reply in due course.

Ron

Yes, I have another project of 52MB that exports fine.

I've just changed the upload_tmp_dir from the default in php.ini to setting it to another location (with more available space free sapce is 66G) in the host.conf file