Cannot login after yesterdays update

Yesterday I got an Hueman theme update notice so that’s what I did. However right after an error starts and I can’t login to my website anymore, get this notice: ERROR: Cookies are blocked due to unexpected output. on all browsers and different devices.

I already checked a lot of documentation such as this yet I could not manage to fix it.

Via the cPanel I switch off all plugins, but it makes no difference so it appears that the error is not plugin related. Technically I am not very experienced, some help with this will be much appreacted!