even if the thrust levers are set to the FLX/MCT detent if no flex temp has been inserted in the MCDU PERF page the FMA will read SET TOGA...the setting of the temp is mandatory for the FMA to call MAN FLX..
scorpilot when did the ECAM show up ...around which speed?above or below 80 kts? what was the exact caution?
The only thing i can figure out from the infos you provided is that no flx temp has been entered or a wrong flx for the ATOW thus Fadec couldnt give the required N1 or EPR setting needed for T/O leading the FMA to tell you to go on TOGA setting.
My .2$ cent...