We just finish upgrade from 3.5 to 20.2.7 and one data collector shows Polling status Not Connected, I restart services, delete the /installpath/apache.../data folder as see in some articles but doesn't works.
DC time is incorrect
DxNetOps Performance Management 20.2
The time on the data collector (date command) was not in sync with other systems. Work with your linux administrator to correct the time on the system and reboot. The collector should connect with the data aggregator