Can anyone tell me what is going on?
For some reason, Inkscape started to show blur in a very pixelated way, which had never happened before.
But whenever I extract the image I created, the blur looks normal again...

Hi. Sometimes it is a display problem that is solved by zooming. If it persists, you may have touched some of these parameters in rendering. These affect the view and not the render.

My 'filter effect quality for display' was set on 'better', which I turned back to 'average'.

I saved the document and restarted it, just to be sure. 

But unfortunately, it did not help at all. 

Then I don't know. Try to reset the preferences. It's a tip that usually fixes some problems. Preferences/System/Reset Preferences