Hi Stefan! These accounts have been discontinued a couple years back as part of a RUG wide domain reorganisation, I believe. As far as I'm aware, all <username>@ai.rug.nl addresses have been notified in advance to check whether they were still in use. If want to know more about it, I'd suggest to contact Remco Wouts (Remco.Wouts@rug.nl), the system administrator for the Bernoulli Institute (which ALICE is now part of) who has also been involved with this.
1) My claim that it has been communicated properly has been disputed, they may just have taken things down without notice.
2) It hasn't been just a domain reorganisation, but they also wanted to get rid of the ai.rug.nl server for reasons I don't know.