I saw this show up several times for this plugin and on first use to register my Meetup group I got it myself. After a few minutes of research I found a fix that worked for me. I'm posting here in the hope that it will solve this for others. YMMV
The first thing I noticed was that there was no information other than the request with a HTTP code 500 in my access logs. Since I wanted to get PHP to display errors, and I only have issues with this plugin, I modified the wp-meetup/wp-meetup.php file so that any errors generated by PHP would be displayed in my browser. To show these errors I added these two lines near the top above the include statements:
ini_set('display_errors',1);
error_reporting(E_ALL);
With that done, I resubmitted my Meetup information and got this back in my browser (instead of a blank page)
Fatal error: Cannot redeclare json_encode() (previously declared in /var/www/domains/domain.com/sites/fcmg/htdocs/wp-includes/compat.php:65) in /var/www/domains/domain.com/sites/fcmg/htdocs/wp-content/plugins/wp-meetup/meetup_api/libraries/xmlrpc/extras/jsonrpc/json_extension_api.inc on line 59
This told me that json_extension_api.inc had a duplicate function to the WordPress builtin found in compat.php. As a matter of fact each of the json related functions had this issue. I tracked back the use of json_extension_api.inc to a require_once call in wp-meetup/meetup_api/MeetupAPIBase.php on line 146. I commented out this require_once statement with two slashes like this:
// require_once(dirname(__FILE__) . '/libraries/xmlrpc/extras/jsonrpc/json_extension_api.inc');
I then reloaded the page and like magic it all worked exactly as I would have expected it to.
HTH someone out there... and if it does, let me know.
Regards,
Billy-