I'm not sure what's going wrong with your installation, so I think the best way is to do a full new installation of Flatpress like the Upgrade section in the Flatpress Wiki.
See full article:
https://wiki.flatpress.org/en:doc:basic:upgrade
So, please download the newest Flatpress Version 1.2.1 here:
https://www.flatpress.org/download
Next step, unpack the Flatpress 1.2.1 completly and start the Upgrade as described in the Flatpress Wiki:
https://wiki.flatpress.org/en:doc:basic:upgrade - Section "Upgrade including full backup"
Upgrade including full backup
This should be the preferred upgrade method and is suitable for any Flatpress installation. It does not matter, whether the blog resides in your webspace root directory or within a subdirectory. In addition, you have a full backup of your webspace.
1) Download your complete blog using your FTP software
2) Delete blog contents from the webspace
3) Transfer the current Flatpress version to your webspace
4) Change permissions for “fp-content” directory to 777
5) Run setup the usual way (including language selection, menus and plugins)
6) Transfer Flatpress data from your backup to your new installation
Rebuild Index in Administration Panel - Maintain
Important Note:
For security reasons, it is recommended to delete both the “Setup” directory and the file “setup.php” from your web space!
If this doesn't work, so it is possible that your current Webspace Provider doesn't support "https://".
Hope this soved your problem.