Our Hudson server failed, because it couldn’t connect to the web server. However, I didn’t find out about it just now, because it also failed to connect to the SMTP server to tell me about it.
It’s really not a major issue. There seems to have been an NFS problem in the department network; I ran into it on my work desktop on Friday and rebooted. I suspect it’s the same issue here, and rebooting the Hudson server will probably solve the problem.
I didn’t find out about it, though, because there’s some kind of SSL certificate problem that prevented Hudson from accessing the SMTP server using SSL. This is a bit more serious. I tried to figure  out  how to add the certificate to the JRE key store, but gave up when i received the nice informative output
keytool error: java.lang.NullPointerException
I just ended up disabling SSL for SMTP. I’m surprised that it still works, but it does.