Anyone using Local (by Flywheel) as their local server will run into the “Invalid+request%21” when adding sites because MainWP is hardwired with a path to openssl.cnf. The easiest fix is to figure out the path your local server is looking for openssl.cnf and add an empty iconssl.cnf file there. In my case I used the WP-ServerInfo Wordpress plugin which has the path in the php info screen (phpinfo) in the OpenSSL section.
Open /wp-content/plugins/mainwp/pages/page-mainwp-setup-wizard.php add the path there and save it.
Here’s the empty iconssl.cnf file I created:
HOME = .
RANDFILE = $ENV::HOME/.rnd
[ v3_ca ]
This issue could be easily avoided if MainWP hardwired the path to an empty openssl.cnf file in the mainwp plugin includes and avoid this issue for the next person that runs into it - it’s a really unhelpful error message but at the least the forum support helped me resolve my problem, so thank you.