contact form 7, I get redirected to the 404 page

Question

I use WordPress' most recent version. I have a pretty weird problem where, after saving contact form 7, I get redirected to the 404 page. I made changes to the htaccess file.

1. Remove all plugins.
2. Replace the current theme with the default WordPress theme. but are still encountering the same issues. I believe there is a server problem, but I'm not sure what it is.