Warning message "There was an error rendering this rich post."
I just upgraded to v2.8 from 2.6.x and switched to PHP 7.3 from 7.0.
Everything looks good except for a warning message "There was an error rendering this rich post." This message is appearing below most existing posts on the forum. I'm using Bootstrap theme. I get it even when I switch to the new Keystone theme.
I can see the same issue on some posts on this forum too, for example:
What do I need to do to fix this issue?