404 error on “Save Changes” Action for Plugins

When I upgraded to 2.8.4 last night, my ‘options.php’ page seems to have something wrong when I change anything having to do with a template or a plugin. I can’t make any changes. I can navigate around in the admin section and the actual site is still fine, but I can’t make any changes. The 404 error says this: The webpage at https://financialplanpro.com/wp-admin/options.php might be temporarily down or it may have moved permanently to a new web address.

I also installed a new plugin last night (wp-eStore), and so I’m not sure if that might be the reason.

I’m using the Carrington Blog theme.

I’m able to figure out a lot of things as I got, but I’m also new to wordpress, so please keep that in mind. And thanks to anyone who can help!!