At the moment that you try to add a public folder standard target, it fails with an error saying:
"unable to validate the public folder root path"
A newly created test public folder does, in most cases, not have this issue. The issues appear with certain older public folders while others are totally not affected. The problem is caused by some old, nearly "invisible" MAPI properties. Most people are not aware that they existed and where these properties are coming from.
Clear the "PR_IS_NEWSGROUP" and "PR_IS_NEWSGROUP_ANCHOR" MAPI properties on the Public Folder. As these are old MAPI properties that can't even be changed using the Exchange Management Shell, there is no impact on the public folders. The MAPI properties are dating from the prehistoric era of Exchange 2000/2003.
The "PR_IS_NEWSGROUP" and "PR_IS_NEWSGROUP_ANCHOR" can be cleared using
I prefer the automatic method using PFDAVAdmin / ExFolders (depending on the version of Exchange)