@Chillbo: Looks you’re an experienced user, some logs would be fine:
- enable debugging flags (How to submit a useful support request)
- reboot the OSMC device
- try accessing both mount points and/or any content there
- upload the log set
BTW.: I Always use fstab-options " defaults,x-systemd.automount,noauto" with my Synology NAS since years without problems. Perhaps you also give this a try.