The last few days I've had problems with being notified about new posts in subscribed threads. I don't know where the problem is, but usually I get a mail whenever there's a new post in my threads (and yes, I've clicked on the "view new posts" to make sure further posts are delivered). That hasn't happened recently, and I've noticed new posts I should have been notified about when browsing the forum.

Is it a known-issue or a problem on my end? I have checked my account-settings and as far as I can tell, there's no problem there. And nothing has been accidentally moved to my spam-folder either.

I have the same problem!
I signed up to the forum a couple of weeks ago, and I never got my confirmation email. I requested another one, and never got that either!
For some reason I was able to log on and post (I'm not entirely sure how) but I never got those emails, nor notification about posts.
Suddenly today I got those emails, including the backlog of notification emails. But they're all dated for the past day or so! NOT when the posts were posted.
What gives?

It was as issue with the KDE server which had a problem with its mail (SMTP) daemon. As a result, all mail were queued up for sending, but were never delivered.

The issue has been fixed by sysadmin, and the mail queue was cleared, hence.