Web page rendering glitchy/slow - "Missing CSS stylesheet..."

[Ultima Thule]

For some reason pages seem to fail rendering properly and they would come out like the following example attached below (Please disregard the blurred-out parts in the picture, it's intentional).
I'm wondering whether it may have something to do with the occasionally slow response loading the web page.
To be honest ever since the update of the forum software the page loading time have been glitchy, it's mostly loading 'just okayish' quick to some times being rather slow, and more rarely albeit too often hanging up, or loads a not fully proper page rendering.
Other forums also using XenForo don't exhibit these glicthes, I think it may have something to do with some custom setup of DiyA's forum SW and/or server settings that is causing the trouble.

Edit: Shall add this is happening on Brave Browser, Linux.

Upon bringing up the "Element Inspector" (more or less every browser have one embedded), which is executed by:

• right-click anywhere inside the web page and select from pop-up menu "Inspect"

or using any of the hotkeys:

• F12
• Ctrl + Shift + I
• Ctrl + Shift + C

A common occurrence appearing in web pages that have failed to render, is the following error message (See at the bottom part of the attached picture below):

• This page failed to load a stylesheet from a URL

This has happened once today, and twice yesterday, and several times in the past.

 

[Pano]

I think it may have something to do with some custom setup of DiyA's forum SW

Probably. That should change sometime this year. We hop to go to a more standard XF setup.
FWIW, loading is pretty good for me, rarely any problems.