Catching Fatal Error: Subscribe w/o Commenting

I’m getting this if I subscribe w/o commenting:

Catchable fatal error: Object of class WP_Error could not be converted to string in /blah, blah/wordpress/wp-includes/shortcodes.php on line 188

I disabled all plugins and still see this. Odd that my sandbox site works fine. Same theme, Stargazer.

https://www.ads-software.com/plugins/subscribe-to-comments-reloaded/