The reason for this was that there was one wrongly named sourcetype (unsupported character) on props.conf.
Before upgrade to 7.x.x it was worked without any errors, but after update it start to write this message to internal log. It also gives error "WARN TcpOutputProc - Possible duplication of events with channel=source::tcp:10514". That also vanished after fixing sourcetype name.
We have the same issue after updated from 6.6.5 to 7.3.2.
Also our HF -> IDX (cluster) traffic has stalled after that. HF said that IDX stops receiving events and on IDX side the haven't found any (real) reason for that yet. All resources seems to be ok even indexers didn't take events in.