The Office Mode from the Primary Gateway is included in its VPN Encryption Domain.
According to the Remote Access Clients for Windows 32/64-bit Administration Guide E80.72 and Higher, page 62:
"If you use Office Mode IP addresses, make sure that the IP addresses are different on each
gateway so there are no conflicts. The Office Mode IP address that is issued by the first gateway is
used to access the secondary gateways."
The issue is that Secondary Gateway receives the packets with a source IP, whose network is configured behind the Primary Gateway.