Hi @marinorama
Thank you for the update.
Wordfence is fully compatible with all versions of WordPress 5 and all versions of PHP 7 so this is something unique to your website.
Please carry out the following:
1) Login to WordPress in your normal internet browser.
2) Rename the Wordfence plugin directory as follows to its correct name:
~/wp-content/plugins/wordfence
Wordfence should already be active on the WordPress Plugins page but activate it if you need to.
3) Try logging into WordPress in a different internet browser.
4) If the login fails in the different internet browser then clear all cookies.
5) You can test to see what is causing the problem by using a maintenance mode plugin. ?You can run the tests on your theme and other plugins and your site visitors will see a maintenance mode page rather than a broken site as you switch themes and deactivate plugins.
We strongly recommend that you create a full working backup of your site file system and database first. It is also a general recommendation from www.remarpro.com to backup your site before installing a new plugin or making other major ?changes.
Install and activate a popular maintenance mode plugin that is compatible with your version of WordPress.
6) Now switch to the Twenty Nineteen theme that you have installed and deactivate all plugins on the WordPress Plugins page except for Wordfence and the maintenance mode plugin.
7) Try logging into WordPress in the other different internet browser. If the login is successful then logout. This means that your theme or another plugin is causing the problem.
You can test switching back to your normal theme and activating all other plugins you use one by one until the login fails in the other browser to see what is causing the problem.
In the other browser remember to delete all cookies after you test logging in.