I upgraded the Drupal core to 8.3.1 today morning. I also upgraded the modules like AT Tools 8.x-1.1, CAPTCHA 8.x-1.0-beta1 etc. on our production site.
Now, the "Main page content" block does not output the node content and outputs the following instead:
< data-history-node-id="46" class="ds-2col-stacked-fluid clearfix"> <> <>
The error is not displayed if I "Disable attaching Fields" from Home > Administration > Structure > Displays : Emergency settings. I think it is an issue with Display Suite.
How to troubleshoot it? Here is the info from the "Emergency Settings" page:
Disable attaching Fields
"In case you get an error after configuring a layout printing a message like "Fatal error: Unsupported operand types", you can temporarily disable adding fields from DS. You probably are trying to render an node inside a node, for instance through a view, which is simply not possible. See http://drupal.org/node/1264386."