GPG4Win 3.0 beta 299, Outlook 2016 32-bit 1705 build 8201.2171 click-to-run as part of O365 ProPlus.
Set GPGOL settings to:
Sign new messages by default
Select crypto settings automatically for reply and forward
Search for OpenPGP keys when encyrpting
With Kleopatra not running, choose "New email", any recipient, and Send.
Kleopatra will be started, and Outlook will hang. Kleopatra does not present "select signing certificate" window. Waited for a few hours, Outlook does not recover.
Kill Outlook in Task Manager, restart. Kleopatra is now already running. Repeat test, signing is successful.