I had the same issue with motion trying to access a synology share (samba in my case).
For me the solution described here did the trick, except that the umount-mount sequence did not work. I had to reboot instead.
And of course I used “uid=motion,gid=root,file_mode=0770,dir_mode=0770” in the fstab entry. Changed it with notepad++, not sed, but that shouldn’t matter.