Okay.
I don't know why this fix worked, or well, I do a little.
However my older RPi 2 had set it's SMB versions to range from SMB1 to SMB3. That worked without any problems.
However it appears to have been a little different with my newer RPi 3 B+. I had to explicitly set the Samba Min version to SMB1 and Max version to SMB1 and enable "use legacy authentication" or something like that, in the same menu.
And suddently it worked!
I really need a NAS so I can avoid spending half a day on figuring stuff like this out. Anyway I will now get on to use my RPi 2 as a Central Database.
I hope this helps someone else out there with the same issue.