The Synology does have the capability of be an Apple Filing Protocol (AFP) server which is Apple's native network protocol. File Manager can also connect via AFP using GVFS virtual filesystem. Probably more information then desired.

I've never tried using AFP since I don't own any Apple PCs so not sure where to look. To manually connect via samba the URL to enter in file manager would be
smb://NAS_device_name/share

A further quick update. I loaded the same docx files which would not open when on the nas drive, onto a usb stick. LibreOffice reads them fine from the stick in the RasPi, so maybe not a LibreOffice issue after all.
David

Possibly a windows ownership/permissions issue. The pi will access it differently than the windows machine, docx is a windows format which implies it originated from the windows machine, and copying the file to a usb stick changes the ownership/rights attributes of the copy (especially if the usb stick is vfat instead of ntfs). It could easily be a windows vs LO issue.

Can you try copying the file from the nas to the pi just to see if it can be read? If it can and the copy can be opened with LO then it definitely appears to be a windows issue with rights/ownership.

Thanks for the suggestion. I have copied docx files directly from the nas to the RasPi (from the File Manager on the RasPi) and they then open without difficulty in LibreOffice. I will try originating a docx file in LO and then transferring it to and from the nas drive to see if that works too. Any other suggestions welcome.
Best regards
David

Could be a permissions issue depending on how the NAS is mounted. Libreoffice should create a lock file in the directory where the docx file is located on the NAS to prevent the file from being edited by another instance. The error might be due to the lock file not being created or the file being opened as read only.