You might face a situation during an Exchange Server migration where your Exchange Server 2019 mailbox users are not able to open their public folder favorites when using Outlook on the Web (OWA).
When your users try to access a public folder, they receive an error message.
This error occurs when the public folder mailboxes are still hosted on a previous version of Exchange Server. This includes Exchange Server 2016 and 2013.
The online documentation explains, why this is happening:
The solution to this problem is easy. Move the public folder mailboxes to Exchange Server 2019 before you migrate any user mailboxes.
This approach ensures that mailboxes hosted on Exchange Server 2019 and previous versions of Exchange Server are able to access public folders using Outlook on the Web.
Enjoy Exchange Server.