Can't edit .htaccess (causes error 500)

Hi.
I’m trying to edit the .htaccess to block some referral spam but I’m unable to make any edits to the file.

I can find, download and reupload the file itself, but if I upload a version with any modification, I face the Error 500 (Internal Server Error) page.

And I really mean any modification, even deleting a comment or a blank line.

Any ideas on why this is happening and how to prevent it so I’m able to make modifications to it?