We have a client that is no longer able to view logs and click on messages that have been sent. The functionality is gone and it was present before. They upgraded from Moodle 3.5 to 3.9. I have a screen capture to show what the functionality used to be. 

Log into a site

Go to Logs 

Click on the hyperlink in logs to view a message viewed by a user

The message will not be available for viewing

You will only see the message if you log in as the sender or the user