The Mixed Content error can appear as a result of some actions in Workbox. For example, the page cannot be refreshed after clicking buttons for moving the item forward through a workflow or when changing the number of displayed items per page and clicking Refresh. Users might experience the message similar to the following one:
Mixed Content: The page at 'https://site/sitecore/shell/default.aspx?sc_lang=en' was loaded over HTTPS, but requested an insecure resource 'http://site//sitecore/shell/default.aspx?xmlcontrol=Workbox&mo=preview&reload=1&{190B1C84-F1BE-47ED-AA41-F42193D9C8FC}=0&{46DA5376-10DC-4B66-B464-AFDAA29DE84F}=0&{FCA998C5-0CC3-4F91-94D8-0A4E6CAECE88}=0'. This request has been blocked; the content must be served over HTTPS.
The problem occurs when Sitecore XP tries to reload the Workbox via HTTP but it is blocked in the environment if only HTTPS is allowed.
To resolve the issue, download and install the patch compatible with the affected product version found on this page: https://github.com/SitecoreSupport/Sitecore.Support.212515/releases
To get a solution for other releases, contact Sitecore Support.