Allowing the cookies did not changed anything to my problem.
But my problem, dating from months and months ago, has just disappeared. And I know when and (partly) why. Suddenly, my browser went crazy, opening a new tab each time I tried to modify a post, thus I could not modify anything, and it was almost useless for anything else too.
So I closed it completely, instead of letting it open as I used to do everyday when I shut down my PC (an hybrid tablet). When I reopened that browser (Opera), the new issues had disappeared, but also that old one of losing my logging at any time without any reason. It's so good now that when I open a new tab (near the first one) and set it to a fresh new HF Underground page, I am also logged on that new tab. Amazing !
So the recipe is : close the browser window completely, with a click on the 'X ' in the upper right corner. Don't expect that just shutting down the PC can refresh the browser, the browser will restart with the same amount of old data.