I suppose you are using a self installed ubuntu system? A typical problem is that there are missing one (or more) PHP module(s).
Yes, it's a VPS server with ubuntu
If you use the Showphperrors plugin as a substitute
Showphperrors shows the following error when I click on a tag:
Notice: Undefined index: ns in /var/www/dokuwiki/lib/plugins/tag/action.php on line 82
I took a look at the line 82 and we can see there the following string: $ns = trim($_REQUEST['ns']);
So, I suppose I won't be able to resolve it on my own because I am totally rookie at that. I rely on you guys