This error happens when Pi-hole is unable to load the stats before the timeout (previously 30 seconds but now it is 5 minutes) closes the connection. We did increase the timeout and memory allowances, but if your Pi-hole is on an older version you may be running into this issue. We will also be changing the error message that displays to make this clear.
You can remedy it in the short term by flushing the log file, but then you can't view the stats for that day.
We are working on a few things to aid this problem. First is our FTL API. Second is a database backend, which is still in the works.