I want to draw your attention to the T511396: Web Dashboards - ArgumentException - An item with the same key has already been added issue, which is already fixed in XAF for v16.2.7. 

This error can occur if the application model contains different BO Models with the same Caption and is visible in dashboards.

If you came across a similar issue (check the call stack), the highlighted thread contains a simple temporary solution (make captions different or remove the VisibleInDashboardsAttribute from classes that are not required in dashboards)  + hotfix installation.