Can not connect to Windows network share from non domain computer via VPN

I just solved the problem. I will summarize some notable aspects:

• Used a different login. Using name@domain schema. The permissions were not okay. It's actually a set of DFS shares.
• Re-Enabled the NAT. Some folders were still inaccessible from a different subnet.
• Created a stub zone (with forwarder to an A DNS server) for the remote domain (A) inside (B).
• Manually configured DNS suffixes. First the domain of the remote net (A) then the local (B)

Now hostnames, FQDN and IP can be used to access Red.

Update regarding the system error 2240

This is a separate Problem caused by the user account. The account has been configured to allow login only from specific Computers. It worked as soon as Blue's hostname was added to the user. This is done in the AD. See unrelated question Change list of allowed logon computers from batch file.