Weird problem for this forum in Vivaldi

Hi, I have a problem with my default browser Vivaldi on this forum: there is an invisible preloader-image that prevents all clicks on the webpage.
I am using it with firefox currently. On Vivaldi, after having cleared all the site data (F12 > Application > Storage > Clear site data (including third-party cookies) it works for the login procedure. But afterwards, it was “locked” again.
It is possible to manually delete the preloader-image in the DOM tree and then it works. But it always reappears…