If all CRM reports calendar controls stop working suddenly and are stuck "loading..." in Dynamics 365 CRM Onpremise Version 9.x , read the article for a solution

 When you click a Calendar on the SSRS reports in Dynamics 365 CRM and does not see date pickup panel but only "loading", it is about; 

Default CRM organization has recently been disabled. Activate it, all start working perfect again.

Technical Details,

If you open the  Developer Tools in Chromium based (Edge or Google Chrome) and look at Console tab, You will see error mesages from 

Reserved.Reportviewer.....datepicker.js  

that state;

Uncaught ReferenceError: HideUnhode is not defined at onLoad (Reserved.ReportView....)

and a warning about 

Resource interoreted as Stylesheet but transferred with MIME type text/html...

in Microsoft.Reporting.WebFomrs.calendar.css.

on the other hand, 

CRM Engine already throws an error and records message under EventViewer at Front End Server which contains meaningful information about the root cause of the issue. 

good luck...