iTunes could not backup the iPad because an error occurred

I have just started receiving this error:

"itunes could not backup the ipad because an error occurred"

when I attempted to make one today. I googled and have tried the following already:

1. Restarted iTunes, restarted computer, restarted iPad

2. Found the old backups in the Applications Support>Mobile Sync>backups

- made a copy to the desktop

- deleted the older backups via the iTunes preferences>Devices option

3. Changed the USB cable connecting the iPad to my Mac to the original from Apple

4. Successfully backed up my iPhone with the cable and iTunes (so it does not look like a hardware issue)

5. Tried backing up the iPad to the computer and iCloud but get the same error on both.

My last successful backup was run a few weeks ago, made just before I updated my iPad Air to iOS11. I have not tried a backup since then. Interestingly, although iTunes says the backup failed and shows the iPad now as never backed up, the attempt to run a backup did produce a folder in Applications Support>Mobile Sync>backups that is 3.4GB in size with the appropriate time stamp. So it looks as if a back up is being run but does not finish correctly.

Any suggestions would be gratefully received. I would hate to do a factory reset and then manually restore everything, this would be a massive pain!

Mac Pro 3,1 running El Capitan 10.11.6 with iTunes 12.7.4.76

iPad Air with iOS 11.3