I have linked the “dietpi_userdata” Music folder to Nextcloud Music folder, and also the same for the Video folder. For making this link, just do (is the same for the “Video” folder):
cd /mnt/dietpu_userdata/Music
And then type:
ln -s /mnt/dietpi_userdata/nextcloud_data/admin/files/Music/ Music
Now, I was having problems with PLEX accesing those folders. The problem was that the “plex” user was not in the group www-data, so I typed:
usermod -a -G www-data plex
Rebooted, and all done
Hope it helps someone, or you could even add this tips in the “Official Manual” for making things easier to everyone.