Since a couple of days ago, I do not receive notifications about new posts to subscribed topic. Before that sometimes a notice would be sent which pointed to old (long read and answered) posts. What's going on?

I would say check spam filters and the like. I just got a notification for this topic creation and my email is not queued any differently than other user's notifications.

My spamfilter isn't blocking dumpshock, I already checked. The topics also show up as subscribed in my control panel.

I recently got notifications from two topics. I guess it is working again.

It should be noted that a lot of factors outside our control affect delivery times. The outgoing mail queues are almost low. I have looked into it a few times because I also get annoyed out the variance of delivery times. Once we upgrade the board, I will have access to a plethora of new troubleshooting tools; Looking forward to that. While it may not resolve the intermittent issue, it will at least give me a better look under the covers.