Jitsi was unavailable after the reboot, because one of the web-interface configurations was still referring to an older certiticate.
The certificate for jitsi.science.ru.nl expired last week and was replaced, but a sub-service was still using a copy of the older certificate. Users were able to open the frontend, but not able to start a meeting. The problem was resolved by updating the copy of the sub-service.
Due to performance tuning having gone wrong the jitsi.science.ru.nl conference rooms cannot be joined by more than one person at the moment. Solved by reinstalling server.