First - I did NOT select the the addition of that registry key as the "solution" or "answer" to this problem.
This "solution" is Microsoft's official work around until they issue an update that will solve the problem and as you know doesn't work for all users.
For reference, these are the suggestive work arounds:
1)The added ReopenWindowsOption registry key doesn't stop the popup window for all users. BTW: That key was already in the Registry in Outlook version 2305 to store the startup option that the user selected in Outlook's options. The upgrade to version 2306 removed the options and registry key which caused the popup window to start being a problem.
2)Some have suggested that that opening and closing Outlook will stop the popup, but other users say it didn't work for them.
3)Other users say that having Outlook settings to save it settings in the Cloud stops the popup, but there are other users that do not save their settings to the Cloud and do not get the popups indicating that setting may not affect the popup.
4)One method that is 100% effective in solving the popup problem is to stop updates and revert to Outlook version 2305. Wait till Microsoft releases an update that fixes this problem and begin updating again. You must have a Click 2 Run version of Outlook.
5)I have never gotten the popup message when I open Outlook, even after automatic updates. I do not save any Outlook settings to the cloud. I do not keep any emails on the mail server (delete all after downloading). I did not add the registry key after it was removed by the update. I reverted to version 2305 and the registry key was added by the update to a prior version. I then updated to version 2307 (by-passing version 2306) which removes the Startup Options on the General page of Outlook's Options, but the registry key and its setting remain in the Registry.
The choice is yours to try.
Another option that might work is to open Outlook Offline and see if the popup message occurs. After Outlook is open, click the Offline button to toggle to go Online to get your emails. Opening and going online at the same time may be triggering these popup boxes. If you decide to try this, the following link is to the instructions for how to open Outlook Offline/Online.
Open Outlook Offline