Please deactivate all the plugins not related to the theme, and switch to the parent them if you are running the child one and provide your site admin access to the private area.
You could add a customization code to the child theme which could influence. You need to work on the parent theme or clean your child theme from the harmfull code.
I think I found the problematic function. I’ve used this to add a new endpoint in My Account page.
I’d rather not delete it since I need this new endpoint.
Is there something I can modify to not make it break the post page?