Moved this to a new thread to prevent the other thread from growing too large with another issue.
This issue was solved with a simple server restart. It’s a known issue that after some length of time (TBD), a server restart is needed for bulk import to resume working. We suspect it is related to how Excel files are opened on the server.
After the restart, I am now able to see the preview window for the 1-line spreadsheet, and the data looks good (except for the missing photo that I don’t have).