When you attempt to Restore any element from SourceOne Web Search, the Restore window stays in a Loading a state and not retrieves the mailbox folder tree.
You may also see the following error displayed
"The Error: Sys.WebForms.PageRequestManagerTimeoutException: The server request timed out."
The cause for this issue is related with Outlook 2016.
In order to resolve this issue you must install Outlook 2016 upgrade from February 2020.
Link to update:
https://support.microsoft.com/en-us/help/4484250/security-update-for-outlook-2016-february-11-2020
Note: If you are using Mixed Exchange environments review below article.