Several things could be causing this error:
I updated the site a friend. We went from 7.1.4 to 7.3.4 via the upgrade stream. Problems started after the cumulative upgrade v7.3.0.
During the upgrades my FTP went very slow with speeds between 15kb and 30 bytes a second. Other than that, the upgrades went smooth enough.
I got so many errors, I also tried creating a new 7.3.4 site and moving the upgraded database across. Although this worked fine, these errors continue to appear on both sites.
On reading about similar errors on the Internet, it could be a server issue, but maybe not. If Nathan is reading this, it's on one of your servers (Loire).
Can anyone throw any light on the subject?
The script continues to work in a way. Thumbs come and go and the template is about 30cm down the page until I clear the browser cache and I can't log out. It happens with all templates.