If HTTPS is activated once, but then it is decided to use the unsecure HTTP protocol again, in more recent browser versions it was observed that the HTTP connection failed in such a constellation caused by a policy change relating to “strict secure cookies” (also see [Chrome Cookies]).
To re-enable an unsecure HTTP connection, clean the cookies of the belonging URL via the browser settings (e.g. in Chrome via the internal settings path “chrome://settings/content/cookies”).