Been using 1.5.3 (6341) for a few days and currently trialling it for our social network site. So far it's a winner! BUT .. I suddenly have issue with logging in (sign in does not work but sign up does) and any page customization (such as changing text). As soon as I click the offending button, the screen greys out and nothing happens. Pics show a before and after 'clicking' 

Only thing I have done so far is edit images and update a few settings in admin area. Not made any code changes.

To try and resolve I have disabled all plugins, and removed all test accounts, leaving only admin account. Java fully up to date. This site is online but not live while I test.

Any thoughts gratefully received.