you say you've made no changes, but that's unlikely.. some things that change pretty subtly:
-video drivers
-windower/ashita/plugin updates
-game client itself
i'd start by running a file check(it's available on the first screen of playonline, delete one of your rom files first to ensure a full check against the server)
if that turns up no results or the results don't help your problem, and it's something you can make happen frequently, attempt to make it happen on vanilla POL
if that's not possible, consider making a crash dump and letting someone take a look:
https://github.com/Windower/Issues/wiki/Creating-crash-dumps-with-Windbg
crash dump won't necessarily turn up useful information, but an informed viewer can identify whether it's a windower/ashita/plugin issue or a base ffxi issue, and possibly whether it's to do with graphics drivers or something else if it's base ffxi