It happens to me regularly. The "what's new" tab is fine, but once I try to get to the inbox, it hangs.
There are no console messages, but the Safari Web Content CPU usage jumps to 100% and memory is eaten fast (14.2Mb every 2 second, crept up to over 7Gb before I force-quitted), according to Activity Monitor. Then I restarted Safari and got into the inbox immediately.