Notifications have stopped in some workflows and for some users after server upgrade

At the backend of 2023 I moved M-Files server to a new system. Went well. Everything worked, people were happy and the IT guy was the hero. Yay!!!.

I then upgraded the server to 23_11_13168_7 in 1st week of January and thought everything was going well until the last week when suddenly a couple of users no longer get notifications to their emails and some workflows have decided to stop notifications via email at certain points. 

For example:

- a Workflow has 4 steps where notifications were working fine. 

- at step one, it should send notifications to 3 users. It sends to 2 users, the 3rd user is completely ignored. Logs on both m-files and exchange show the sending for 2 users and receiving for 2 users. 

- at subsequent steps, it just doesnt send emails at all. 

I have checked and double checked and triple checked notification settings, the workflows and users. 

Suddenly the IT guy is looked upon again as the villian in the office  :( 

Any suggestions here short of recreating brand new users and workflows.