That's quite unexpected. Could you attach the exact file itself here? If it has sensitive data in, you might be able to upload it to a file sharing site and private-message me here a password.
I'm thinking first along the lines of the code not being in a standard module, but in a sheet's or the workbook's module. The line Set RngToCheck = Range("D13:AB63") has an unqualified reference and it may not be the same as the ActiveSheet of the subsequent line.