After any change on presentation page, "Page not found" message appears

Hi, there!

Every time I change anything on the presentation page (say, for instance, a slider image) after clicking “save setings” it takes me to an error message: “Page Not Found”. After that, the whole site crashes, and it takes several minutes, sometimes even hours, for it to get back.

I have tried everything (deactivating plugins one by one to rule them out, change the name of the image the file I was trying to add to the slider, etc.) but I keep having the same results.

I just updated Tempera, and the issue persists.

What can I do? Any ideas?