Can’t edit theme, edit menu disapearing

I would like to update my website visuals and can’t edittheme (that menu on the left which appears) I opens and as soon as it is loaded it refreshes page and menu is gone.. tried on Chrome, Firefox and Explorer… same situation.
see video I made of what happens:

thought it is problem with theme, but no, I installed also other theme and have same issue. I have other website and it is working all perfect. so must be bug in this particular one.
WP latest version updated
no plugins installed recently,all updated
tried that same theme (Brunelleschi)on other site, all is ok

any ideas?
thanks!