I managed to recreate and solve the problem this time by being more careful for every step I take testing it (and testing reboots after I change stuff). The cause was my /tmp setting which was like so:
After setting it and rebooting I encountered the problem again, I unmounted /tmp and samba worked again.
This was a copy paste job I forgot to actually test properly, so my bad (although I wasn't aware that samba relied on /tmp, how does samba use /tmp?)
Things work gain with a more simple fstab option
Since Samba is the most important function of the NAS for me, I need to know exactly what it is using /tmp for and why it fails without it. (Once I know this is resolved).