=== POTENTIAL FIX ===
Hi all, I found a simple workaround that worked for me.
What seems to happen is that the plugin periodically decides there’s a problem with the MaxMind data and deletes the entire geo file (maxmind.mmdb). From then on there’s no geo reference, so all the data comes back empty.
Every time I re-uploaded the maxmind.mmdb file, it got deleted within a short time by the plugin.
To fix this, I uploaded the Maxmind file again as per the plugin instructions and then made the folder “wp-content/uploads/wp-slimstat” not-writeable (permissions octal 505). This stops the plugin from deleting the geo file, and hopefully it won’t cause new problems.
So far so good. Hopefully it will work for you too.