hey @nickh
hmmm that sounds like an SMB permission issue. is the SMB share mounted as admin and made available for the users via external files? try adding the same share per user in external files, with each users SMB credentials.
otherwise mount the SMB share into the host via fstab as root, and then connect to the hosts local share via external files…
you’re not using the snap that’s clear, but this may help understand: How to add samba shares and local shares to Nextcloud snap