New CSV File Upload Issue

For many years, every month, we've generated CSV and/or Tab Delimited files and have had no issues uploading them. Starting this last month we receive an invalid file format error. If I open the files in Excel and resave as a CSV they will load. The files I uploaded successfully last month, which we haven't touched, give us an invalid file format error. What has changed? Has anyone else experienced this and have you been able to mitigate? Thanks