The "Kopano Outlook Extension" (KOE) plugin must not be activated if GpgOL is active as this leads to mails not checked by the GpgOL plugin.
The plugin icon stays purple with a questionmark. Seems like it never gets the content of the email.
As soon as the KOE is deactiated GpgOL generally works again.
Maybe this is something for the list of incompatile add-ons here: https://wiki.gnupg.org/GpgOL/IncompatibleAddons
I will try to also report this incompatibility to the Kopano maintainers.
See attached log where an encrypted and signed mail was sent from one to another account also added in Outlook and then opened while the KOE plugin was active.