Cannot save file, error in writing sub document content.xml

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Bought a new laptop with Windows 10. I've loaded Libre office on it and started work on a writer file I've been using on my old windows 7 machine. Been ok for a few days and saving ok. Then, today, this message turned up when I tried to save Error saving the document Toxic Waters: Write Error. Error in writing sub-document content.xml.

The message turns up when I; save, save as, save as (doc), copy onto a new document and save, copy on a new document as unformatted text and save

It doesn't save my work.

I have the up-to-date download of Libre office (and I've tried the previous one, still the same). The file works on my, less portable, computer running windows 7. Which would be great but I need it on my windows 10 laptop.

Please help Simon