I finally had a crash again today, when I tried to close outlook.
I was running the debug log for several days now, until it finally crashed. Using gpg4win 3.0.1.
Here from the debug file:
*removed entries for privacy reasons*
12:28:46/19196/oomhelp.cpp:remove_category: category 'GpgOL: Verschlüsselte Nachricht' not found.
12:28:46/19196/mail.cpp:parsing_done:882: tracepoint
12:28:46/19196/mail.cpp:parsing_done:885: tracepoint
12:28:46/19196/gpgoladdin.cpp:gpgoladdin_invalidate_ui: Invalidating ribbon: 1D363A70
12:28:46/19196/mail.cpp:parsing_done:900: tracepoint
12:28:46/18768/parsecontroller.cpp:~ParseController
12:28:46/18768/mimedataprovider.cpp:~MimeDataProvider
12:28:46/18768/attachment.cpp:~Attachment
12:28:46/18768/mimedataprovider.cpp:~MimeDataProvider
***here I closed Outlook, but Outlook froze. I then killed the process in Windows.
12:34:02/19196/windowmessages.cpp:gpgol_hook: WM_CLOSE windowmessage for explorer. Closing all mails.
12:34:02/19196/mail.cpp:close_all_mails:1084: tracepoint
12:34:02/19196/returned from invoke

Updated this morning and just had a freeze again. Enabled debug mode now. For me the freeze often happens when my pc is locked and I'm not using it for a while, then return to my PC and select another email.

Tested Beta 305 which was more or less fine, today installed Beta 307 and have problems again. The problems today are so far only Oulook hangs, which resolve after around 15-30 seconds. Sadly once it started with the first hangs, it hangs a minute (or so) later again and I can barely change the mail. Those are all plain mails, without any encryption/signature.

I'm having the exact same issue, also Outlook 2016 and Beta 299.
[edit]
I want to add, my Outlook also often freezes by just changing the folder. Outlook will try to open a message when chaning the folder, but regardless if it's encrypted or not, Outlook might freeze.
I did not change any GpgOL default settings.