Thanks for the help, Timo! It turns out the skin was causing the problem(s). Now everything works and looks just fine. 😀
We're continuing investigation to the performance issues, you can follow and react here: https://lemmy.world/c/friendicaworld
I haven't installed any extra extension that weren't in the original install package (I'm using MediaWiki 1.45.0-alpha).
Below is a screenshot of the search result page (sorry for the meta search term, btw). As you can see, there's no option to create the missing page. Just a "No result"-messsage.