I ran into a new issue that I wasn’t aware of until now. When Exchange 2013 was RTM’ed, it was known that OWA no longer supported Search Folders. Those are the folders you can create in Outlook that show a specific filtered view of your mailbox. Those can contain items like UnRead Items, Missed Conversations, Call Logs, Flagged Email, Categorized Email, etc… I personally don’t use Search Folders, but I know that many people do. I also know a few organizations that have standardized their desktop infrastructure using one brand or another of VDI. Within their VDI solution, they have chosen to deploy Outlook 2010 or 2013 in Online mode and their internal Exchange 2013 servers have been scaled to support Online Mode.
This lays the groundwork for the problem. Outlook cached mode, along with many other features, is this mode that maintains the finder info on the local PC. Since the finder information is no longer synchronized to the Exchange 2013 server store, the server doesn’t store finder information any longer (which indecently is one of the reasons why Search Folders are no longer offered in OWA). In Online Mode, the users still can try to use Search Folders, and they appear to work until they try to interact with the messages shown in the Search Folder. Actions like Delete will produce an error message that tells the user (depending on Outlook version) that either the message can’t be found, or better yet, “Unknown Error”.
The solution is to enable Cache Mode, full stop. I have not yet confirmed this with the Exchange Product Team but it is my belief that since Office 365 utilizes Cache Mode exclusively and since cache mode is now supported in VDI deployments, that this is by design.