Blank page -> no redirect to /login

Hi, for some time there is no redirect to the loginpage, only an empty page is shown when I open the URL. Then I add /login to the url and there is the loginpage. When logging out there is a message Access is prohibited and CSRF check failed. Also the message "Problems loading the page, the page will reload in 5 seconds.
My system is:
Ubuntu 16.04, Nginx 1.10 ,php7.3-fpm, mariadb 10.2, redis 5.0.6, nextcloud 16 and 17, same problem…
I tried php7.2, no improvement.

Does anyone knows the problem?