It's not clear where in your query this error is occurring. Let's break it down into the smallest possible parts. Is your Source stage connecting to the SharePoint folder still working? Screenshots would help.
Thanks, there is nothing outside of basic data query functionality that I am doing here.
I create a "Sharepoint Folder" data query with a sharepoint connection that works fine for .csv file combining, but fails with these Excel files.
I filter to the folder in Sharepoint (I have tested this with a fresh connection to Sharepoint numerous times). Then use the "Combine Files" feature in the content column. It creates the helper query and converts each file (they are all identical excel files and only excel files are in that folder).
It errors when I just refresh the flow, and I can trigger the error in the dataflow editor when filtering by string values: