FYI I think there is something wrong with imports since 4.0.3 update. I have spent hours to get customers imported, the site would repeatedly reject some records without obvious reasons. Even when it is down to “E-Mail” single column ONLY, it will reject the import.
Workaround:
Now I realised that when you set to All Stores, you get Skip errors, then set to either Store and it works partially, then to All Stores again and it works fully. This is on CS Cart Ultimate, two storefronts.