Once upon a time there was several on premise mailboxes. The follow up flags were working flawlessly from Outlook running in “online mode” on a RDS server. Someone installed hybrid mode and moved the mailboxes to O365. Now the user was really disappointed with the response from Outlook still running in Online Mode. One of the users got archive mailbox enabled. The primary mailbox was moved to on premise. We now got a new issue : It was possible to tag messages with flags, but they never appeared on the task list. Where do you start to troubleshoot this? Our first check is to see use OWA. Usually this will give us a hint if it is client or server side error. And … it looked faulty in owa. So this seems to be a mailbox error. What to do next? We tried to run Outlook with command line parameters – in this case /resettodotab . And….. that did not help. Next we tried to create new outlook mapi profile, but that did not help either. Then a more drastic approach – we did a export and import of the entire mailbox from Outlook. During the import we did select to overwrite existing elements. We also selected to start Outlook using the same command line /resettodotab. This time it worked out fine. This was a time consuming approach so now we tried to move another mailbox, with the same problem, to another mailboxdatabase and that also worked fine. Conclusion for us was to move the troublesom mailbox to another mailbox database and start Outlook with command line /resettodotab, We did not try to uninstall any office patch , since we had this problem on both Outlook 2010 and Outlook 2013. This worked fine for small mailboxes (>500 MB) but not for larger ones.
Ended up creating a supportcase with Microsoft and the result of this is that this is an issue. Senario is when a Exchange 2010 mailbox is moved to O365 (Exchange 2016) Legacy attributes are removed from the mailbox. These attributes does not exist when the mailbox is moved back to onprem (Exchange 2010) and there for issues like this will emerge.
Why does it help to move a small mailbox to a new database? According to MS support : when a small mailbox( less than 500 Mb) is moved to a new database a new content table is created, this is not the case when you move a larger mailbox.