I had the same issue last year. I followed the below steps to fix that.
But it does not help me as my ports are enabled.
2) Copied the the 1st License key from Settings --> Licenses tab of Tableau Server.
We can see multiple licenses if you have purchased it long back. Select the oldest License Key from the list.
3) Copy it to activate on Tableau Server through Offline Activation. After restart of the server the Maintenance date got changed for me.
You can find the Offline Activation Steps here : Activate Tableau Offline
Hope this works for you too.