/login vs. index.php/login

I am running a private Nextcloud 15.0.11 instance on a virtual server.
Unfortunately my users keep complaining about login issues, i.e. that on seemingly random occasions they just can’t login and are redirected back to the login page.
I recently encountered the same problem and have narrowed it down to only happen when logging in on the “$nextcloud_instance/login” URL. There’s no error message and no log output. The user is just redirected to the same login page again.
However with the URL “$nextcloud_instance/index.php/login” everything is working fine.
Can someone explain the different behaviour between both?
I suspect it to be an issue with URL rewriting but I can’t remember to have set anything up to do that.