The next step I take in these instances is to re-create the import from scratch. Upload a brand new file (give it a unique name) and make sure the file is in plain .txt format. Re-map everything. If you're still getting an error, I would take that file and remove all but one column, and attempt the import again. If that works, I would add one column at a time until you come to the column where you're getting the error.
One other thing to keep an eye on is the time scale applicability and range in your model. Make sure that if you're loading monthly data to a line item, that the line item "Applies to" months, and that the months associated with the data you're loading are included in your model's time scale (e.g. if you're loading FY18 data, your time scale supports FY18).
Most of these suggestions are stating the obvious OR recommending you check things you've likely already checked, but it's an obligatory step when trying to troubleshoot things virtually / not live.
Let me know if you're still having trouble and we can hop on a quick phone call.