Those messages are matching a couple of internal PRs of MX platform.
According to PR notes,
This seems to be a timing issue, where RE has asked stats for a particular IFLSET but this particular IFLSET is not present when PFE is processing this request. Looks like PFE is processing this request after IFLSET is gone. This is only seen when number of session is more than 500.
Can you try running the versions mentioned by user "randero" and check if the logs disappear. With respect to why the error is seen and the meaning of it, this seems to be internal data. The information given by randero should be helpful for you in getting rid of the error.