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The .htaccess file in your site's root is generated by Admin Tools. It is not "misconfigured", and does not have "many mistakes". Moreover, the fact that nothing changed after removing it proves beyond any reasonable doubt that it had never anything to do with your issue.

I don't have enough information to tell you what's wrong right now, but I suspect that a third party plugin may be the root cause. Edit your configuration.php file in your site's root and find the $debug and $error reporting lines. Change them so they read:

public $debug = true;
public $error_reporting = 'maximum';

Reproduce your issue. You will get a longer message with a backtrace. Copy the exact message you get, including the backtrace, and do tell me if it happens before or after you try to log in. I can help you further having those two pieces of information.

This tells you that the current user does not have permission to access Joomla's Users component. I have a few questions, though.

Are you trying to log into your site as a Super User, or a lesser-privileged user. In the latter case tell me which user group you belong to.

Have you enabled any features which force the user to edit their own account upon login e.g. forced MFA, or Joomla's com_privacy? If so, have you tried logging into the frontend of the site using the same user to see if you can apply the necessary changes to your user profile there?

Is your site behind a CDN, a TLS terminator, a load balancer, or an opaque caching proxy (e.g. Varnish) which may be caching the response from a different user trying to log into the site?

Before going down a rabbit hole, do you perhaps have a recent backup of your site from a date you know it was working fine?

I am diving deeper into that, even though it's definitely out of scope of our support – mainly because I am very curious as to what is actually going on, and whether I can troubleshoot a weird issue without having hands on the site (if I didn't like a good puzzle I wouldn't be a software developer).

It looks like you are using Joomla 5.4 and the problem comes from Joomla's Multi-factor Authentication (MFA). The line number referenced in your message says that you were authenticated (username and password), then you are taken to the MFA, which rejected your request as it considered itself impossible to display.

My problem with what I am seeing in the stack trace and your description of what you did – especially the part about restoring a backup of a known-good site – is that they do not fit. The only reason the specific code line referenced in your stack trace would return the 403 is if you have disabled all multifactorauth plugins, or if your user is simultaneously in a forced MFA and a never MFA group. However, either of these issues would've made logging into the original site as that user impossible.

What I would do is restore the site, then use phpMyAdmin to remove all entries from the #__user_mfa table, thus disabling MFA for all users everywhere. If this doesn't help, also find all #__extension table entries where folder = 'multifactorauth' and enable them.

Since you never tried any of my suggestions all I can do is ask you to follow my suggestions.

Ooooh! The way I read your previous reply I understood that after the site was working you had restored it again with the same problem. OK, no worries, your site's working now, all good.