In fact, with sufficient perseverance, it would also be possible to block yourself from accessing /c/portal/login (as from Liferay 6.0.6). This can be done by setting the community homepage to a hidden redirect page (as opposed to the default portlet page), as a result of which / c / portal / login is redirected wherever the home page is redirected.
Liferay tries to prevent this by issuing warnings when you try to hide the home page. But you can make some page hidden, and then drag and drop it to the first position on the root level of the site map.
The reason for this is to achieve a certain type of portal navigation (which otherwise would require some distortion in the subject), i.e. make home page redirection one of its children.
Returning to the topic, when /c/portal/login fails, there is another, more reliable (but even more ugly) way to access what can be called a login page - add this ?p_p_id=58&p_p_state=maximized to any unreferenced Liferay page (and possibly remove any other GET parameters, if they exist). This is also basically what /c/portal/login does, but adds it to the home page, so if the home page is a redirect page, it does not display the login portlet in the most compressed state.