Alright, so how exactly do I fix up the .htaccess?

Did you try reinstalling the script? It still wont work for you even if you install a fresh copy? If so, you may need to submit a ticket to your host and see if they can get anything out of it, this looks like a host-specific issue to me.

I reinstalled, and now it's working. Except that I can't login with my admin username and password -.- Should I just keep reinstalling until it works, or...?

Well try clearing your cache to see whether it works. If not, clear your cookies too. II had a user with the same login problem before, she fixed it by the steps above.

Did both, and it didn't work... I know I'm using the right password and username...

Umm this is weird, can you check if the user actually exists in ur database? If not, this may as well be the problem. Otherwise, just try to reset your password by clicking the forgetpassword link and see what you get.

Indeed, it says there are no registered members on the memberlist.

Just as I expected, the admin user was not created properly. Chances are you did not empty your old database before installing the script. Well anyway, try to register a new account and then make it admin user by going to phpmyadmin table prefix.users. Edit the field usergroup to 1, and you can access ACP.

Yay, it worked!

Thank you very much for all your time and help!

My pleasure, glad it finally works for you. Tbh I am still kinda wondering why some people are getting the class appcontroller not found error while this can be easily fixed by simply reinstalling the script lol.