I had the same issue with (no message syncs since 7/31) using Message plus on a Windows 10 pc. I just updated to Windows 10 ver 1903 and started using Microsoft's phone connect application instead. My messages sync like before. I'm not sure Verizon will update their app or there must be a new conflict with Windows 10 that recently started and is only cured using Microsofts Your Phone app. Hope this helps somebody!