No output from shortcode

I have configured the plugin with CC login and created a form with shortcode [constantcontactapi formid="1" lists="1"]. I added that to a page. That one page is now very slow to load, and there is no output from the shortcode. The only message I get with debug on is: “Notice: Array to string conversion in /var/www/…/wp-content/plugins/constant-contact-api/functions.php on line 490

Any suggestions for debugging it?

https://www.ads-software.com/extend/plugins/constant-contact-api/