Tableau Server can become unlicensed because the ATR activation is not being refreshed due to a proxy server.
Proxy server configuration was not completed correctly and cannot connect to the ATR service.
It is possible to verify if the ATR lease is being refreshed by running ATRDIAG -product "Tableau Server". The TTL START and TTL END will change every 24 hours at a minimum. See the Tableau Community Blog on Server ATR for more information.
Tableau Server ATRDIAG Reference Guide
Netsh Commands for Windows Hypertext Transfer Protocol (WINHTTP) | Microsoft Docs
Configuring Proxies for Tableau Server
Note: regarding the installation of Tableau Server, system variables (like http_proxy and https_proxy) for proxy server would also be required. Please confirm the articles below for more details:
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