There's a thread in this forum about bugs in the code release. After my client had been attacked, I analyzed, what exactly happened and figured out, how the attacker managed to get the clients to basically crash. JLP and CfB fixed that now mostly by 0.5.3, the only thing that is left is the actual browser page. So when you see negative blocks, just refresh the tab in your browser and everything will look normal again. On the server side (communicating with other NXT clients) the server now behaves normally, even if you don't refresh your browser window.
If you want to see what kind of bugs we're finding and that are getting fixed, jump over to
https://bitcointalk.org/index.php?topic=397183.00.5.3 been running since released, so far so good. If it's still running OK in the morning that would a new record for me. Can't wait to find out.

Running 0.5.3 via NxtMac, on bad branch?
36406 14571285356259793594 January 9, 2014 2:27:28 AM GMT+02:00
2 0 + 3 305 B
2 5313834224092465353 74.3 %
2901 orphans, 8651 unconfirmed transactions