MixConvert doesn't work in Monitoring Section Nuendo 6.5.35


I have a problem with Nuendo 6.5.35 on Win7.
Since i had update my Nuendo on 6.5.35, i can’t open the mixconvert plugin on the monitoring section.
I click the rightmost button to open the MixConvert plug-in but i just have the list of different downmix possibility ?

Can you tell me if it’s a bug ?

Thank’s a lot