so, its been approximately 15/16 hours since the update: A Letter to GGG

We've collated solutions to common problems related to the update here: https://www.pathofexile.com/forum/view-thread/1595121

We're trying hard to get everyone fixed up before Ascendancy comes out in a few days. This redistributable update is necessary for Ascendancy, and we released it early so that there's time to fix any remaining issues.

If people are still having issues after following all the steps and are desperate to play, my recommendation would be to clean install the most up-to-date version of Windows you can get and then install Path of Exile from scratch. That works very reliably. The newer the better in terms of Windows version.