Are you logged in to your site with the super admin user? Only the super admin user can get into the package manager. Also, if you see the login even though you are logged in as super admin, sometimes it just needs you to log in with the original super admin user for the package manager page.
The only bug I know of that was reported and is on the list to fix is the one I just mentioned. The package manager page sometimes needs the original super admin login. So, if you changed the super admin login email or password, you'll need to use whatever it was originally. That bug is being fixed.
If that is not your issue, then you need to check your hosting account to ensure mod_sec is not enabled, open_basedir is not enabled, PHP version 7.2 is on the server, and that you are not using FCGI on the server. Also ensure that permissions are fine: https://www.socialengine.com/support/article/5112151/se-php-optimized-file-and-folder-permissions
This site uses SEPHP 5.5 and I have no issue accessing the package manager.
Thank you for posting the solution as it could help others.