Well, also a "Newbie" could know how the system works - it's just like a "Title" for posters here in this forum.
As written, OpenCart per default has only 1 xml-file which is called:
modification.xml
It is stored inside the folder
../system
Means, if you have more xml-files in this folder something goes wrong.
Second: in the backend under
Menu > Extensions > Modifications you will see installations of other modifications - thies data is stored in the database.
If you are now not able to start the backend, simple open your FTP program and
delete all files inside the folder ../system/storage/modification
This will clean your system back to an "unmodified" system.
While you have the FTP open, check for other files inside the ../system folder (as described above).
Now you can open the backend.
To avoid the same situation, do not refresh the cache under
Menu > Extensions > Modification (Button Gear top right)!
If you do that, and it happens again, do what described above again.
While I do not know what these installers like
softaculous are doing, never using them.
Better download a fresh copy of the OpenCart version you want to use from here:
https://www.opencart.com/index.php?route=cms/download
and install that (delete before the old installation completely).