This the most common error message and it's related to incorrect queries.
Reasons:
In the widgets; you are trying to connect fields from the tables that are not connected.
There were updates related to one of the fields used for the widget creation.
Solution:
Hover on the message and see what fields are disconnected. Make necessary changes to ensure that only connected fields are used for the creation of the widget.
If all widgets on the dashboard have this symbol; something could be wrong with the dashboard filters. Toggle all dashboard filters OFF. If the errors disappear; find the filter that causes the problem by turning them back on one by one. The next step; remove the filter that causes the issue.
In rare cases; e.g. if a field was renamed or after the upgrade; it might be necessary to remove and re-add fields in the widget designer to find which field causes the problem.
Go to the widget's edit mode.
Starting from 'Categories' remove and add back the field. Continue removing and adding field by field until the issue is resolved in the following order 'Widget Filters'; 'Values'; 'Break By'.
Dashboard is not found
This error happens when the last viewed Insights dashboard can't be found.
Reasons:
The last viewed dashboard was deleted or unshared by the dashboard owner.
Solution:
Pick one of the available dashboards on the left pane.
If no dashboards are available; create or import a new one by clicking '+' button on the top rigth corner.