Well, this version didn’t last long.

Apparently a bug in version 7.5.1 for Safari caused XKit to not set proper “header” data while posting data to Tumblr servers, causing some functions like One-Click Postage to not work. This version fixes this problem. 

To upgrade, please visit the XKit website and re-download XKit.

You do not need to uninstall XKit if you are on XKit 7.5.1 (you need to remove it first if you are on 7.4 or below, though.)

My apologies for this.