FR24 have just tweeted that they have good coverage in that area, with planes in contact all around where they 'should' have been. However ATC contact is not lost for another 1hr 20mins.
So either are something happened to the transponder at 17:20UTC which was part of a chain of events leading to loss of contact 1hr 40mins later OR this is unrelated transponder issue either onboard or with the ADS system FR use.