I had the same problem and the solutions provided were not very helpful because manually reviewing hundreds and, possibly, thousands of Names is not something that is simple to do.  What finally made it so much simpler to find the errant characters in my QB lists was to use the Reports, Lists menu option to view all of the Customers, Vendors, Employees, and Other Names as reports.  When viewed this way any "unsupported" characters show up as garbled in the report output.  It turned out, in my case, that I had an accented e for a customer from Canada who has been in our QB for years.  When I changed the accented e to an "e" the error disappeared. 

Had I not seen the list of customers as a report, I would have never found this problem because I thought the error had to with something that had been recently added to QB. 

Hopefully, this is useful for somebody else.