we are having one primary, secondary and arbiter in replica set so we are recieving errors in our secondary instances so what causes the errors and is there any risk factor to our cluster what are requirements to stop these errors please find the attachment of errors and please give solution.

Log File Monitoring : key = error, File = /var/log/mongodb/mongod.log (host11:log[/var/log/mongodb/mongod.log,@mongodb_error,UTF-8]): 2019-01-20T02:03:02.174+0900 W REPL [rsBackgroundSync] Fetcher stopped querying remote oplog with error: InvalidSyncSource: Sync source's last applied OpTime { ts: Timestamp 1547917372000|605, t: 60 } is not greater than our last fetched OpTime { ts: Timestamp 1547917372000|605, t: 60 }. Choosing new sync source.
Log File Monitoring : key = error, File = /var/log/mongodb/mongod.log (host14:log[/var/log/mongodb/mongod.log,@mongodb_error,UTF-8]): 2019-01-18T01:57:58.646+0900 W REPL [rsBackgroundSync] Fetcher stopped querying remote oplog with error: InvalidSyncSource: Sync source's last applied OpTime { ts: Timestamp 1547744268000|2966, t: 58 } is not greater than our last fetched OpTime { ts: Timestamp 1547744268000|2966, t: 58 }. Choosing new sync source.
Log File Monitoring : key = error, File = /var/log/mongodb/mongod.log (host11:log[/var/log/mongodb/mongod.log,@mongodb_error,UTF-8]): 2019-01-15T03:52:13.002+0900 W REPL [rsBackgroundSync] Fetcher stopped querying remote oplog with error: InvalidSyncSource: Sync source's last applied OpTime { ts: Timestamp 1547491922000|3629, t: 60 } is not greater than our last fetched OpTime { ts: Timestamp 1547491922000|3629, t: 60 }. Choosing new sync sourc