When connecting to Tableau server data sources from Tableau Desktop, returning error: CFC8C9BB

Published: 04 Nov 2022
Last Modified Date: 07 Nov 2022


Tableau Desktop keeps prompting "Reconnecting to data source" when loading the dashboard with data from multiple tables located on Tableau Server.  In addition, the following error can display during this connection failure: 

An error occurred while communicating with the data source 'data source name'.
The connection to the data source might have been lost.
Error Code: CFC8C9BB
Malformed response from Tableau Data Server.


  • Tableau Desktop 
  • Windows 10
  • Tableau Server
  • WAF firewall


  1. From Tableau Desktop log find the following log statement: "This is the malformed resp '<html><head><title>Request Rejected</title></head><body>The requested URL was rejected. Please consult with your administrator.<br><br>Your support ID is: xxxxxxxxxx</body></html>
  2. Provide this Support ID to the security team and check the firewall block log.
  3. Find the block details and adjust firewall rule and policy.


Tableau Desktop's "execute query" request is blocked by the user's web firewall as their firewall has a rule to block the "join" statement.
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