/splay has worked before on this version of mIRC (6.35)
That statement alone eliminates mIRC as being the problem.
It's not possible for /splay to be working one day and not the next with the same version of mIRC.
I recently installed some Windows Updates via Automatic Windows Updates
If this is the only thing you have changed since /splay did work, then it (likely) is the cause.
Try removing the updates, then check if /splay works again or not.
If it does, then go through the updates again, one at a time, installing the update, then checking if /splay still works or not.