Hello!
I didn't get any suggestions, but finally I found the solution myself so I'm writing it here for any users with the same problem.
The solution was that even though the user existed on the host system I needed to add it to the samba server aswell. This is done by typing:
smbpasswd -a username
And then you just enter the password you want (twice), and restart the server with:
smbd restart
And it works. I'm still having problems with permissions, but now I can connect atleast.