If you are having trouble with adding external storage mounts in Nextcloud, you can do this:
$ sudo -u www-data php occ files_external:list
You can see the bad external mounts and delete it with:
$ sudo -u www-data php occ files_external:delete X
Then, you can re-create good ones by using this command:
$ sudo -u www-data php occ files_external:create Blabla ‘smb’ password::password -c host=X.X.X.X -c share=X -c root=/X -c domain=workgroup -c user=X -c password=X
You can add applicable users to the mount using this command:
$ sudo -u www-data php occ files_external:applicable --add-user=X
Replace: X with Yours.
Reference