We migrated InfoPath forms from SharePoint 2010 to 2016, some of the forms used a data connection to ExcelServices.

THE PROBLEM:
When InfoPath tries to connect to Excel Services using a string to the Office Online Server we got this error “The SOAP response indicates that an error occurred on the server: We couldn’t find the file you wanted. <detail><correlationId xmlns="http://schemas.microsoft.com/office/excel/server/webservices">3ab4d1aa-a4f7-48e5-9c83-9efea4c2a8fc</correlationId></detail>”

What…?? it looks that InfoPath is looking for a file on the OOS server to connect to but cannot find it.

THE SOLLUTION:
ExcelServiceInternal.asmx endpoint is not meant to be used directly from any clients. It is reserved for SharePoint. In order to be able to connect to Excel Services, you need to use the ExcelServices.asmx service on SharePoint 2016 which acts as a proxy for OOS: https://SharePoint.domain/_vti_bin/ExcelService.asmx

(The SOAP request must have this format for the URL).
– Big thanks to Philip & Luc for all the troubleshooting