What happens if you preview in a different browser, such as Firefox?
If Safari is set as your default browser, you can preview in Firefox with the following steps:
Preview in Safari
Copy the URL in Safari’s URL bar
Paste it into Firefox’s URL bar
Edit: Sorry, I re-read your message and I see that it’s not that the link isn’t working but that the page doesn’t preview. It’s possible to change your default browser (e.g. from Safari to Firefox) in the Desktop and Dock pane of System Settings (or Preferences on older versions of macOS).
Something that might cause this problem is if Safari doesn’t have permission to access files in a particular location (e.g. your Documents folder).
Thanks, Jeremy, for your quick response. It seems that Safari doesn’t talk to the recently updated Dropbox. Changing my default browser to Firefox fixes the problem because Firefox has the option to allow access to files in Dropbox - Safari doesn’t have that option (at least I can’t find it!). So for now I have a solution of sorts.
I looked at Reload Page but it’s greyed out so that’s not an option.
I’ll let you know if I find any way of persuading Safari and Dropbox to co-exist.