I have three subfolders of my Inbox: 2016, 2017, and 2018 where I store "done" emails.
I noticed that I'm unable to drag-and-drop my 2016 folder, and when I view the context-menu in Office Outlook 2016 the "Move folder" menu-item is disabled, but the 2017 and 2018 folders are fine.
2017 and 2018 folders:
Why is this? And how can I move my 2016 folder?
In the status bar, Outlook reports "All folders are up to date." so I don't think it's an issue with a folder being locked while it's synchronizing.