Dear,
I found the problem. It’s caused by the plugin Wordfence. I tried with & without the plugin, it’s precisely this plugin that makes the 403 Error issue.
Can you please test it on your side and fix the compatibility of your theme with Wordfence because, it’s a freemium plugin used by million of WordPress users.
Right now, I’m doing my changes in my header while temporarily disactivating the plugin. But it’s important you fix this issue or if it’s a problem on Wordfence side, then take proactive action and contact them to fix it.
I can provide you the detailed Wordfence setting by in private if you give me an email.
Thanks