DCS 800-SO2-450-5 warning " ConvOverTemp"

We are using DCS 800- SO2--450-5 DC drive for pinch roll application in rolling mill. During material rolling, the drive some times was giving warning ConvOverTemp.
We have checked that cooling fans are working & cooling is normal.
Also par 1.4 BridgeTemp is not going more than 33 Degree Celcius. We have checked PTC resistance value i.e. 2.018 KOhm.
When Bridge Temperature is normal , why drive is giving warning ConvOverTemp. Please let know what exactly is going wrong.