You changed now all modes recursively, but this is probably difficult to revert, as especially Nextcloud creates very individual modes 
. But not a big problem, webserver R/W permission is guaranteed. You may want to remove execute permissions from files:
find /media/SSD -type f -exec chmod -x {} +
It would have been sufficient to change only the parent dir mode, non-recursively:
chmod 0755 /media/SSD
Background is that for general access for an unprivileged user, it needs to have at least execute permission for all parent directories. And your original /mnt/SSD mode was 0750, so only www-data user and group had access. Btw, for the parent directory, root user and group should be fine, so the webserver and PHP cannot mess with what they are not supposed to touch 
:
chown root:root /media/SSD