Dataset Refresh Error: 431 - "Request Header Fields Too Large."

. Based on the troubleshooting, Fiddler trace and the error, below are the points which will help you to resolve or reach out to Salesforce for your issue:

1. On fiddler I have studied the request headers being sent from your Power Bi to Salesforce and the header has 16470 lines of request being passed to Salesforce.
2. The limit set by Salesforce is 16,000 characters and 1000 for the API call to be received so that it can process the request
3. The limit is 1000 for the number of calls being made and the character length limit is 16,000 characters for salesforce
4. This can be seen on their post as well: apex - How to fix error 431 Request Header Fields Too Large . HTTP CODE[431] in Anonymous Window - Salesforce Stack Exchange
5. The iDataReader interface connects your .Net framework to pass the request to the datasource and if the datasource doesn’t respond with the request then it will throw the error
6. In summary, you would have to reduce the number of columns that you are writing or adding in your M-Query to resolve this issue
7. Once the characters are less than 16,000 characters the issue would be resolved as Salesforce would be able to comprehend those many characters