Get a working client
If the game will not let you log in since September's retail patch, the website now has one page that walks you all the way from a fresh install to a working launch — and the one trick that makes PlayOnline actually fetch the update.
Since Square Enix's September patch, the most common question in the Discord has been some version of "my client will not update and I cannot get in". The website now answers it in one place: Get a working client.
It covers the whole path — installing the free retail client, forcing the update through, the launcher, and the three things that usually go wrong after that. Most people only need the middle part.
The short version
1. Check what build you are on. Open the PlayOnline Viewer and click Check Files on the left. It names the FINAL FANTASY XI version. You want Ver.30260904_1. The launcher's top bar shows the same number, from your last launch.
2. If you are behind, update. With a PlayOnline login, just start FINAL FANTASY XI from the viewer and it patches before the title screen. Without one, Check Files on its own will not update you — it only verifies the version you already have. Give it something to repair first: close everything, rename ROM\0\0.DAT to 0.DAT_old inside your FINAL FANTASY XI folder, then run Check Files and choose File Repair. It fetches the whole September patch. Check the version line again afterwards.
3. Update the launcher. Take the update when the title bar offers it. You want 0.33.0 or newer — that is the one that keeps Ashita's own files current, which is what brings item names back in the Market, your bags and the Warehouse.
That is it for almost everybody. If it still refuses, the page has a symptom-by-symptom list: the Defender false alarm on the game loader, the PlayOnline "repairing" loop, the loader signature error, and why Verify Game Files sometimes claims a thousand files differ when nothing is wrong.
No Square Enix account and no subscription are needed to play here. If you get stuck, bring your build number, your launcher version and the Launch Log to the Discord.