Getting this error after updating to the latest version:
This block has encountered an error and cannot be previewed.
WP 6.4.1 version. Site cache cleared. Happening on multiple sites.
Please switch to another theme and see if the same thing happens then.
It also might be caused by using an old type of widgets. Some versions ago, WordPress changed widgets.
Install this plugin: https://wordpress.org/plugins/classic-widgets/, and see if the problem persists.. It brings back the previous widgets' display and construction.