Whole subdomain 404 after using Membership Pro 2

Not sure what just happened to my site... I created a new site from my Multisite admin dashboard (a new subdomain of my main site).
I used the Membership Pro 2 plugin to set up a standard membership to require login to that subdomain. Membership was created, and as I was clicking through the settings tabs, I was redirected to an Internal Server Error (which unfortunately I did not screen capture).
After refreshing, the subdomain is now a 404, page does not exist.
Now, if I try to "Add Site" (using any subdomain name), I get this error message (but it does still add the site)
BUT if I try to add the same subdomain name as before (that I deleted), it shows the error message, adds the site, but site still redirects to 404.

I'm really confused and it'd be great to have help resolving the "Add Site" error message (that displays when adding any site of any name) as well as the 404 page does not exist error (which reoccurs even when I delete and recreate the specific subdomain)