That link is not really correct. “contacts” right before export is not a fixed string. It has to be the ‘uri’ of the according addressbook. This might be different from “contacts” due to localization (see rakeknivens comment above) or naming of additionaly created addressbooks. The ‘uri’ can be looked up in table ‘oc_addressbooks’ in your Nextcloud database (prefix might be different from ‘oc_’ in your installation). So the link should be:
https://[SERVER]/remote.php/dav/addressbooks/users/[USERNAME]/[URI]?export
Though the download also does work with an additional slash in the URL, I would leave the slash out. The download-link of the contacts-app doesn’t have it either.
There is also the script calcardbackup which has the advantage that no cleartext passwords have to be stored on the server and added calendars/addressbooks will be backed up automatically without the need to adapt a script.
Cheers, Bernie_O