Will you please tell us
(1) how you are connecting to the internet and to WordPress.com. The way you connect to the internet (mobile, satellite, DSL, dial-up) and to your blog, and how many proxy server jumps etc. it takes to connect can cause problems. There maybe be problems with your ISP and/or with the proxy servers. Also note that the main dashboard isn’t designed to work with touch devices through the tablet and smartphone browsers.
(2) exactly which browser and version of it are you using. If you don’t know click here to find out > http://whatsmyuseragent.com/
(3) exactly what happens when you try to assign sub-categories to posts?