(Prio high for internal reasons)
Without the Microsoft VBA for Office Addin Outlook no longer has the "Explorer.PreviewPane.WordEditor" object. This causes GpgOL not to show the signature state in the message list as it can't figure out which mails are currently selected.
This might also explain additional problems which have been reported and were not yet reproducible.
I do not know how the Addin can be disabled. I didn't find a way through the GUI or the Registry. Maybe it's possible through Group Policies?
For testing I just renamed the
C:\Program Files\Microsoft Office\root\Office16\ADDINS\OUTLVBA.DLL