There's a recurring log entry every few minutes that I don't know how to fix

There’s a recurring log entry every few minutes that I don’t know how to fix.

Nextcloud version (eg, 12.0.2): 18.0.1
Operating system and version (eg, Ubuntu 17.04): ubuntu 18.04
Apache or nginx version (eg, Apache 2.4.25): 2.4.29-1ubuntu4.12 amd64
PHP version (eg, 7.1): 7.2.24-0ubuntu0.18.04.3

The issue you are facing:

Is this the first time you’ve seen this error? (Y/N): Y

Steps to replicate it:

  1. Open the log

The output of your Nextcloud log in Admin > Logging:

[index] Error: Exception: Argument 1 passed to OC\Core\Controller\LoginController::tryLogin() must be of the type string, null given, called in /var/www/nextcloud/lib/private/AppFramework/Http/Dispatcher.php on line 170 at <<closure>>

0. /var/www/nextcloud/lib/private/AppFramework/App.php line 125
   OC\AppFramework\Http\Dispatcher->dispatch(OC\Core\Controller\LoginController {}, "tryLogin")
1. /var/www/nextcloud/lib/private/AppFramework/Routing/RouteActionHandler.php line 47
   OC\AppFramework\App::main("OC\\Core\\Controller\\LoginController", "tryLogin", OC\AppFramework\ ... {}, {_route: "core.login.tryLogin"})
2. <<closure>>
   OC\AppFramework\Routing\RouteActionHandler->__invoke({_route: "core.login.tryLogin"})
3. /var/www/nextcloud/lib/private/Route/Router.php line 299
   call_user_func(OC\AppFramework\ ... {}, {_route: "core.login.tryLogin"})
4. /var/www/nextcloud/lib/base.php line 1008
5. /var/www/nextcloud/index.php line 38

POST /login
from at 2020-03-07T17:04:35+00:00

I recall two reports on GitHub:

Not sure where the error is coming from.