2019-08-14T21:00:48Z ERR State for READER-006.log should have been dropped, but couldn't as state is not finished

HI, Filebeat team,
we have lots of error lines in the filebeat log, i am wondering is this serious? what might have caused this? thanks!
2019-08-14T21:00:48Z ERR State for READER-006.log should have been dropped, but couldn't as state is not finished.

thanks!
yan