Random error: image instead of page

Hi,

I developed this site with Grav: https://www.escuelametodista.cl

I'm getting the following issue: sometimes a page will load a random image instead of the actual page. I already cleared the cache several times.
Usually reloading the page fixes the issue, but it's unpredictable and not at all user-friendly. I also changes the favicon of the page with another random image.
I'm getting this error since Grav 1.6

It looks like this:

Should be like this:

Thanks in advance for any help