I managed to solve all my problems. Here are my notes to the article K2 and upcoming changes to Exchange Online authentication.

1. My Autodiscovery setting in ConnectionStringEditor is always False

2. In [K2 installation path]Host ServerBinK2HostServer.exe.config in the "system self" tag I have the K2 service account listed as domain "K2:MYDOMAINK2_Service" including the K2 security label, because I don't have the AAD security provider.
When I left the account here as UPN - "K2:K2_Service@mydomain.com", I was not getting notification of the assigned task and errors appeared in the Hostserver log like:

"64007 Provider did not return a result for K2:K2_Service@mydomain.com on GetUser"
"1 The From Address is empty or missing"
"61007 Permanent failure sending message New Task: System.Exception: No destinations could send the message New Task.

3. I have SmartActions enabled, in [K2 installation path]Host ServerBinK2HostServer.exe.config in the tag "system self" I have enableListeners="True" and SmartActions work. When I want to disable SmartActions, I change this key to False and restart the K2 service.

4. The settings mentioned in the article as "Solution Steps: K2 Five 5.0 and K2 blackpearl 4.7" work for me in K2 Five 5.5 too. That is, unless I'm interested in some more integration of K2 with Exchange Online for tasks and appointments, etc., and just need to send email notifications.

5. The article doesn't mention how to request an OAuth token, after all the previous things I set up and now I'd like to test it to see if it works. What helped me was the test workflow with the Send mail and Task steps that I ran and the new process fell into Error where the message was:

OAuth token is expired and requires reauthorization.
User: K2 Service Account (display name of my service account in AD)
URL: long URL with "&prompt=admin_consent" at end

I copied that URL and opened it in a browser, logged in as a user with Global admin rights in O365, and confirmed the requested rights.
The token is then visible in K2 Management -> Authentication -> OAuth -> Tokens.
Then just click on Retry above the process in the error and it runs again.

6. I didn't have to go through the K2 reconfiguration when I was originally set up K2 to use SMTP and wanted to switch to Exchange Online, just follow those steps. If you still go through the K2 reconfiguration, you have to set up OAuth again in the ConnectionStringEditor, which will disappear during the reconfiguration.

7. In the ConnectionStringEditor, the "OAuth Resource" parameter must always contain the OAuth Resource name for the Exchange Online integration created in K2 Management, never the name of the App you register in Azure AD.

8. In [K2 installation path]Host ServerBinK2HostServer.exe.config in the sendmailfrom property I have the email address of the K2 service account, the same as the one in O365, not some "the Azure admin email address" as mentioned in the article.

Hopefully these notes will help others when setting up sending mail to Exchange Online and OAuth.

Thank you @PavelS , I have the similar case.

I use K2 Five that is integrated with Exchange online with OAuth. I had follow the article K2 and upcoming changes to Exchange Online authentication.

Email notification was successfully sent by using “Send Email”, but “Task” notification didn’t work.

I got the following error message:

“The From Adress is empty or missing”.

“System.Exception: No destinations could send the message”

After I set enableListeners="false" in K2HostServer.exe.config, the “Task” step successfully sent email notification.