Plugins effecting layout & login

Hi all, I’ve been working on this issue by myself for a week, and I feel like I’m at my wit’s end. Need some advice on what to try next.

The issue is that my site has the white screen of death (no login) when either the WordPressImporter plugin is turned on, or the plugin Members Access (by Chris Abernathy). This is not the only issue though.

Also when either or both of these plugins are turned on, the css for my site is messed up. The nav is pulling in completely wrong, and certain areas have incorrect fonts. This leads me to believe there’s more of an issue than two plugins fighting over a js file.

When I turned on the debug in wp-config, I get the following error:
Notice: add_option was called with an argument that is deprecated since version 2.3 with no alternative available. in /mnt/stor1-wc2-dfw1/469547/756542/www.cefgreenville.com/web/content/wp-includes/functions.php on line 3497

Any suggestions as to what I should try next?

Also, two notes: I didn’t build this website, I’m just managing it. And the issue was present before I upgraded WordPress and all my plugins. I updated in hopes there was a bug fix in one of the apps, but updating has changed absolutely nothing.

Thanks!