Problems with mail notification for subscribed topics?

In the German subforum there is a discussion whether the mail notification for new posts in subscribed topics / forums may be broken:

Apparently some users receive mail notifications. Others don’t receive these mails, even if it had worked until a few weeks ago (about middle of July). According to their own statements they haven’t changed anything in their settings, and a local spam filter can be excluded.

Also, some subscribed topics / forums don’t show up in the subscription list in the user control panel. Deleting a subscription and newly subscribing don’t change this.

Any ideas what might be causing this?

It works now (since yesterday?), but what was the problems?