Flask keeps loading old version of HTML template
This is very strange but it was solved by RESAVING base.html. 
Though base.html was on server, when I did some changes to it and resaved, Flask started to load the new version of index.html.
I have no idea how this worked.
I changed my index.html template so that it would extend base.html. And also changed the content of index.html a little. But Flask keeps loading unchanged version of index.html
I've tried following:

Deleted index.html temporarily. Flask started to complain jinja2.exceptions.TemplateNotFound: index.html
The same happens if I change the name of file to be used here return render_template('index.html')
To make sure that Atom editor really applies changes to the file on my server, I opened the file with nano over ssh. Changes are really there.
Cleared browser cache. Tried different browsers. Old version of html template is loaded.

I have no more ideas. 
Do you have any idea what else should be checked?