New OneDrive client doesn't sync directories with a '#' in it's name

While # is technically now supported in file names/paths in OneDrive for Business and SharePoint, it's not always enabled by default in your tenant.  This blog post outlines the availablity of supporting it.

https://blogs.technet.microsoft.com/wbaer/2017/04/06/new-support-for-and-in-sharepoint-online-and-onedrive-for-business/

1. Tenants created June 2017 or later should have spport enabled by default

2. If you tenant is older than June 2017 you need to run some PowerShell to enable it.

Once you run that PowerShell, you should be all set.