Haven’t found this to be an Autodesk issue in any of the times it has come up. Usually it is an infosec thing (i.e. your firewall is blocking access), but sometimes it is a conflicting add-in or package which is blocking access.
Two things to try and if neither turn up a result a third step to narrow things down:
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Disable all your Dynamo packages and customizations, and alol your Civil3D annd customizations and see if that fixes it.
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Check your Dynamo log to see if there are any noted conflicts or other issues.
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Partner with your IT team and Autodesk support (usually best to have the IT team member submit the ticket via manage.autodesk.com) to identify which infosec policies or network configurations to see what the blocker is.