Is there any way you can provide the CSV and XLSX files either on the forms of in a PM? If not, I'd recommend calling Esri Support for help. It's hard to say what's wrong without seeing the files.
One thing to try. If you cut out 1900 lines from the CSV, save it, and use that file, does it magically work? If not, then the issue is either a poorly formatted CSV, or improperly referenced external CSV in the XLSX. If it does work, then it may still be an improperly formatted CSV, but the issue is in one of the lines you deleted.