Could be a lot of reasons. Think about routing on network, servers etc.
Or even ACL that has been set on the CIFS server. The server does probably not know where to route the real ip or blocks it.
Maybe it routes it wrong end it ends on the wrong interface of the firewall (then you get anti-spoofing messages or out of state)
Based on the info you shared it is very unlikely that it was a firewall problem and therefore I cannot help further.
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