Migrating legacy public folders (Exchange Server 2010 or older) to modern public folders (Exchange 2013 or newer / Office 365) requires a cleanup of public folders.
There are quite a lot of blog posts and tutorials available describing the general process of migrating legacy public folders to modern public folders.
First you have to identify all public folders having a backslash “” as part of the public folder name.
Get-PublicFolderDatabase | ForEach {Get-PublicFolderStatistics -Server $_.Server | Where {$_.Name -like ""**""}}
Just rename those public folders to a name without a backslash.
Another issue might prevent a successful public folder migration: Access Controll Lists (ACL)
This will be the case in public folder hierarchies that go back to the early days of Exchange and have never cleaned up properly during past Exchange migrations.
The cleanup any orphaned Active Directory accounts
