I am asking for generally checking if all elements of a page has been loaded. Is there a way to check that basically? 
In the concrete example there is a page, I click on some button, and then I have to wait until I click on the 'next' button. However, this 'Next' button is available, selectable and clickable ALL THE TIME. So how to check with selenium that 'state'(?) of a page?
As a reminder: This is a question about selenium and not the quality of the webpage in question....
Reliably determining whether a page has been fully loaded can be challenging. There is no way to know if all the elements have been loaded just like that. You must define some "anchor" points in each page so that as far as you aware, if these elements has been loaded, it is fair to assume the whole page has been loaded. Usually this involves a combination of tests. So for example, you can define that if the below combination of tests passes, the page is considered loaded:

JavaScript document.readyState === 'complete'
"Anchor" elements
All kinds of "spinners", if exist, disappeared.
Is there a way with python-selenium to wait until all elements of a page has loaded?