What happens if you copy the field value from a row that you know works to the cell in Excel that is causing the error? Then does your import work?
I have seen examples where people think the data is the same, but really isn't. Sometimes there are special invisible characters (spaces, carriage returns), especially if the data was copied from somewhere else, like the Web.
But all this is really just speculation. Things like your data, table structure, and the method of import all play import roles here. Without knowing all those details, it is very difficult to pinpoint what your problem may be, so all we can really do is guess.