As 'The_Rock' suggests, sounds like a TAC case, but also worth gathering the below to then submit to TAC.
Ensure your running JHFA118 on R80.40(latest GA release), R80.30 is JHFA228 (May want to consider latest ongoing take which is JHFA236 which has been out almost 3 weeks now so its getting close to GA).
Gather cpinfos from Office A and Office B
tcpdumps from Office A and Office B
Do some VPN debugging on both sites:
vpn debug trunc
vpn debug on TDERROR_ALL_ALL=5
vpn debug off
vpn debug ikeoff
Collect $FWDIR/log/vpnd.elg from both devices.
Ensure MTU Path discovery is working (I think there was a post about MTU discovery and allowing inbound access to ICMP type 3 code 4, but I would whitelist access to the gateways for this rather then generic inbound access to the gateways)
b.t.w Is Office A a virtual system or a physical gateway?
I think with the above you will be giving TAC allot to go on to escalate quickly if required.