Your .htaccess file is huge. It will take some time to analyze and I am currently extremely busy. I will get back to you. It is likely that this is indeed the problem.
In the meantime, you can try the recommended default for WordPress: https://codex.wordpress.org/htaccess
I am not sure if something else will break when you replace all these rules, so keep a backup of your current .htaccess file.
with regards