Hi,
Did you use the Nextcloud “External Storage” app? If so, there is no need to run the file scan and you can comment out or remove that crontab entry.
For external storage you can enable previews in the configuration area. When you click the 3 dots, you see the checkbox there. If this is checked, then it should actually not be necessary to have the cron job for that storage set up.
However I just saw this issue:
The last post could mean, that there is no way to disable preview generation on ext. storages - which would match the topic. In that case you have preview-generation enabled anyway.