Hmmm, don’t ask me how I did it, but it works now. That is, only when I use https://www.hasperhoven.net/index.php/feed/ but that is what the Firefox RSS-button (and Safari too) gives me, so that’s OK.
I was trying the /feed/-URL because the one with the index.php redirected me there before and neither worked.
I’ve been messing around with a few other plugin’s that didn’t seem to work (a tag cloud, snap.com), but I don’t exactly see how that can influence things.
Anyway, problem gone away for now, but I wouldnt’ exactly call it ‘resolved’:-)
