The .logins file just stores the last login and the pages edited. You wont loose your posts, it will regenerate the file the next time you log in. Just trash the accounts folder, and recreate the accounts.
Might be worth using MAMPS gui to reset the file permissions. It seems like something funky is going in with them causing kirby not to be able to write to the accounts folder. Give this a go.
Leave the index.html in there, it is there to prevent indexing of the folder. Try with only deleting the .logins file. If you can’t remember your admin password, delete the admin user account file as well.