You need to make sure your server can reach the WMS server. Is it being blocked by proxy or firewall? You may be able to test this using a browser on the server and trying to reach the map server. We had exactly this problem and it required us to set-up the proxy settings on the server:
Control Panel -> Internet Options
- Select Connections tab in Internet Options dialog and click on LAN Settings
- Set the required proxy.
I had the same issue and echoing what Eric said, you may need to check with your networking folks. We use a reverse proxy and some other process so I had to modify our "online.tms" file as well. I'm not a networking person so I can't offer much help; I had to have Tableau support and our networking guy talk to each other (while I sat and listened) and figure/test things out before I could implement the fix.
Please verify that the configuration of Network settings are correct. The following two articles should be helpful in resolving. Please review the settings and information in both articles.
If the issue is not resolved, please submit a Support ticket to Tableau Support for assistance with resolving this issue.