Just follow the following simple steps to make it work again:

1. Click on the X icon
2. Click on Manage Extensions
3. Click on One-Click Postage 2
4. Select Re-install / Manually Update this Extension

Also, please note that you should’ve gotten the update automatically. 

If your One-Click Reblog was not working, it means there is/was a problem with your Automatic Updates extension, please reset XKit by going to www.tumblr.com/xkit_reset.

Thank you.