It sounds as if you have a malformed csv file. You could try this Automator Service (Dropbox download) to see if that works any better.
To install double-click the .workflow package and, if necessary, click 'download anyway' in System Preferences > Privacy & Security.
To use select the file in Finder, right-click, choose 'CSV to Tabs on Clipboard' under Services, then click once in a new table in Numbers, and control-v to paste.
You can remove it at any time by going to Library > Services (in Finder hold down option key and choose Go from the menu) and deleting the way you would any other Finder item.
SG
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