Tommy,the causes of this type of error are many. It can be the windows operating system, a faulty HD (faulty sectors) or 3rd party software interferring. The primary cause in WX5 appears to be empty cells on a page. Make sure you fill every cell (eg text object), it need not have any content. If rows of cells are not required, delete them (they can easily be added back to the page).

You can totally avoid this error by going to step 5, export project.

Not only does this provide you with a complete and reliable backup, the iwzip can sometimes repair a faulty project.

Never rely on the built in backup routine, it is unreliable and does not include all vital files.

If you can open your project, export it to create an iwzip, then re-open wx5 and import the iwzip. This will not overwrite your existing project and frequently repairs minor anomolies in the project.

When exporting the project, take the opportunity to rename each iwzip (perhaps with date/time appended) so you have a number of fall back options.

Thanks for the reply. I already create a iwzip back up in the manner you suggested and that's what has been saving me from total disaster. I have checked all my site and made sure there are no empty cells as you advised so will see how things are now. Thank you.

Have managed to make several uploads with updated changes to my website without any problems. Still got my fingers crossed but so far the above advice seems to solve the problem. Again my thanks.

That's great Tommy, thanks for the feedback. 

Spoke too soon. Again I'm getting the same error message. Seemed to be working good after I filled in all the empty cells but once again I'm back where I started.

Tommy, how frustrating, perhaps we should rule out a faulty sector on your hard disk first.

Find cmd.exe, right click and run as administrator. Into the DOS box type chkdsk c: /r (if the boot disk is an SSD it is recommended to run chkdsk c: /f) and when prompted say yes and reboot. With luck your PC/laptop will check the hard disk (it should take quite a while, if it does not, please try again).

I have a pretty high end system with a relatively new SSD and large normal HD. Everything boots from the SSD but all data is stored on the large HD. I encountered a number of instability issues a while back (including one error in XML) and after checking both the SSD and HD stability returned.

Since we are playing this guessing game, can we assume that when changes are made that affect multiple pages (eg template structure, header, footer, etc) you do a ctrl+preview to force a refresh of every page?

All of the above is based on the assumption that you are only working locally (not across a network or on an external drive) and you do not have synchronising to the cloud turned on (eg one drive, drop box, etc) or background backup. These can wreak havoc.