Open Document Format such as what LibreOffice writer outputs has stopped being back compatible with earlier versions of LibreOffice and therefore other office programs that support the format. Attempting to read in a file, a dialogue reports that it is broken, offers to repair it, which fails.
That would not be a big deal, if LibreOffice offered me to save in old format option. The issue arises with different Linuxes and Windows. Open Office 7.? And 24.?
Shame, LibreOffice had been very good until now.
I tested saving in .doc and .docx format and as expected this worked fine, but additionally there were no warnings about loss of formatting I had anticipated. In addition, reading the files in Debian LibreOffice there was a warning about a missing hyphenation en-gb which I found in synaptic, installed and this means there are now no warnings.
The worked around is good always save in .docx or another not ODF format.