I'm not entirely sure what's wrong here but it's likely that you don't have all of your Apache and PHP modules enabled. Could you send your error log?

https://pastebin.com/juXpNUnW

I'm not entirely sure what's causing the 404 error but the error on the main page is being caused by not having the fileinfo extension enabled in your php.ini configuration file. Make sure to remove the semicolon before any lines that look like extension=fileinfo.so or extension=php_fileinfo.dll or extension=fileinfo in your php.ini.

I'm having the same issue when setting enable_password_login to true.
@NullRien Did you ever find a solution to this?

I have a theory about what might be causing this and I believe there was a fix for this pushed to the repo, I'm just waiting for @JoeGandy to accept the most recent PR so I can release a new version.

Based on your error log, I’m pretty sure this is actually an error with your configuration and not the uploader. As I mentioned before, I believe the issue is that the fileinfo extension is not enabled. Here’s a guide on how to turn it on: https://stackoverflow.com/questions/3579072/php-fileinfo-is-undefined-function

I hope it's okay if I reply here since I'm having the same issue.
I decided to reinstall PHP (now running on 7.4.11) and checked to see if the modules are loading using php -m

root@PJSalt:~# php -m
[PHP Modules]
...
fileinfo
...

Furthermore, I created a file that invokes phpinfo();
Running that shows as well that fileinfo is being loaded.

So it doesn't seem to be a fileinfo issue.

I found out what was causing the issue for me.
Inside /etc/apache2/apache2.conf I had to change the AllowOverride from None to All for <Directory /var/www/>
This is what it should look like.

<Directory /var/www/>
	Options Indexes FollowSymLinks
	AllowOverride All
	Require all granted
</Directory>

After then restarting apache (service apache2 restart) it has been working without a problem and I'm now able to use the login.

Great, that might be the problem too. However, I think for @NullRien, based on the error log they sent, it's the issue with fileinfo. However they should still check both.