Hi, apologies it was not clear/good questioned.
I deleted my child theme and restarted the whole process. I am facing the same cache error:
The process that I followed is:
- Create a new child-theme using the CLI method. Set to inherit from Future2021.
- In the new child-theme (future2021-child-theme) I copied the same folder structure that pmoreno kindly provided here.
- In assets>css>custom.css I added my custom css for the index.html.twig template, that I want to be rendered for the home page.
- Activated the future2021-child-theme in system.yaml.
- The home page has a “index.md” file with some dummy content. In the grav admin GUI is also set for template:index.
- Loaded and the above error was shown for the home page.
- Loaded another page (ex: localhost:8000/blog) and the page was showing normally correct.
- Changed templates from future2021-child-theme to future2021 (parent) and the home page shows normal with the dummy text.
→ My conclusion: the problem is in the caching of the index.html.twig of my future2021-child-theme - I stop doing things and come to comment it in here.
In another question I posted, @anon76427325 & pmoreno helped me to render both my custom html and css. I had followed the above process and the index page would show just fine.
It was the other day that the above errors started showing up. I wanted to repeat the whole process of the above mentioned comment of pamtbaau & pmoreno to have it as clear as possible. Then the following things happened:
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First, the error was to have a blank page for my home page AND the same kind of cache error when trying to load another page such as localhost:8000/blog. I had done the process stated above but I thought that perhaps I had missed something.
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So, second, I decided to repeat again the process from 0. Then my index page showed normal.
- This is when when I wanted to link the titles of the main page to my other pages (because even though is a custom index page, I want it to be linked to the other pages, so if in index I have a part for blogs, I want it to go to the localhost:8000/blog, following the folder path). In my last comment is this part:
My idea of linking the index page to the ‘wharever’ page started off very primitive, like this: <h1><a href="../../../pages/06.wharever/">wharever</a></h1> . My goal was to "now, let me link each of the titles in my custom template to the actual pages where I have the content (these pages are all following the original future2021 templates). Please, note that perhaps I am doing stuff completely wrong in the linking, this was going to be the learning curve for my day: to learn to template html correctly so that the contents get sourced as if it was another template and not just a chunk of html.
The problem I faced in this part is that the crikey error showed up with the cache error. It was not very related to my “first linking try” in my humble knowledge of things. I was expecting an error related to a twig templating mistake in my syntax or similar…
When researching this cache error is when what I was reading online lead me to think it might be a PHP version error. Sorry I gave it so uncontextualized.
In any case, today I gave it another shoot from 0, as described above, and I got stuck with the cache error. Today no blank page shows up. I don’t know why the blank page was showing, can’t reproduce/remember what’s wrong thing I did in that try… sorry 
Sorry for taking some days to answer, I decided to do it once on holidays because I can devote better hours of the day to think on this (instead of night hours…).
Thanks a lot both of you!!