Thanks for the responses.
Indeed, the issue happens every time the user clicks on "To:"
Something else the user pointed out: when she opens her names.nsf, goes to "My contacts" and clicks on "Browse for contact", it freezes her client.
So, the issue is with that names selection dialog box.
I gave her access to my mail file. Opened my mail file, created a new mail, clicked on "To:"... Notes froze... so it is a problem on her client.
I made a new copy of the names.nsf, closed Notes and renamed it so that Notes will use it as the mail contacts db. Unfortunately, it did not solve the issue.
Made a copy of Notes Preferences, deleted it, restarted Notes and this time it worked!
The user has a second screen connected to her Mac (extended screen).
I realised that if the Notes client is on her Mac's physical screen, it works.
If I drag the Notes client to the secondary screen and click on "To:", it works once. If I click on it again, it freezes Notes. Even if I drag the client back to the primary screen, clicking on To: freezes the client.
I deleted Notes Preferences, restarted Notes and it is working again... We asked the user to keep the Notes client on the primary screen as a temporary workaround.
I looked at the contents of the original Notes Preferences and here are some weird entries:
It looks like Notes is trying to create a dialog box that is way bigger than the screen resolution...
If I compare with values from my notes.ini (I am on a windows laptop, but I also use an extended secondary screen), here is what I see:
Is there a definite solution to this?