Here we are. Got it from Aeroflot pilot's forum:
Description
1) What are the conditions to trigger the "FUEL: ZFW/ZFCG DISAGREE" message?
2) How can be triggered a spurious "FUEL: ZFW/ZFCG DISAGREE" message?
Solution
1) The conditions for the FCMC to trigger the ECAM warning "FUEL: ZFW/ZFCG DISAGREE" are the following:
1- the value of ZFW sent by the FMS differs between side 1 and side 2 of more than 100 lbs,
or
2- the value of ZFWCG sent by the FMS differs between side 1 and side 2 of more than 0.025 %,
or
3- during the initialization/re-initialization process, the status of ZFW/ZFWCG sent by the FMS differs between side 1 and side 2 during more than 2 seconds.
2) Investigation of the spurious cases of ECAM warnings "FUEL: ZFW/ZFCG DISAGREE" on A330/A340 basic aircrafts showed that when a ZFW/ZFWCG is inserted on one MCDU, each FMS will send these values to their onside FCMC with a boolean set to high (to indicate to the FCMC to init/re-init the values) during a few seconds. But, the setting/resetting of the high position for this boolean is not synchronized between both FMSs. A delta of more than 4 s may be observed.
And because of the 3rd condition, the FCMC issues the message.
This FMS/FCMC interface issue is addressed in the FMS "Release 1A" (R1A) standard.