DataSource.Error: Web.Contents failed to get contents from URL (431): Request Header Fields Too Larg

HTTP Error 431 which officially says "Request header fields too large," means the

server has dropped the request. The header sent by the user is either too long or

too large, and the server denies it.

If I understand correctly, the query with this request worked previously, and without

making any changes to the query it now fails when using the Salesforce Connectors that

are developed and maintained by Salesforce.com .

I encourage you to open an Issue at  Issues - Microsoft Fabric Community, and if this is more

urgent to open a Microsoft Support Case.

As a Salesforce customer, you have the ability to Create a Support Case

for individualized support or the ability to search for a related issue

or other documentation. 

Hi danielcontente were you able to find a solution? As a workaround, maybe you can try to test your connection with a 3rd party connector. I've tried windsor.ai, supermetrics and funnel.io. I stayed with windsor because it is much cheaper so just to let you know other options. In case you wonder, to make the connection first search for the Salesforce connector in the data sources list:

After that, just grant access to your Salesforce account using your credentials, then on preview and destination page you will see a preview of your Salesforce fields:

There just select the fields you need. It is also compatible with custom fields and custom objects, so you'll be able to export them through windsor.  Finally, just select PBI as your data destination and finally just copy and paste the url on PBI --> Get Data --> Web --> Paste the url.