This came in via the feedback form:
The apostrophe in my surname is usually the cause of these problems but basically the link to my user profile appears as /user/1.0157433998421E+16 which looks more like the result of a mathematical equation than a user ID.
When I try to access my profile, I get a "Page not found".
I was able to reproduce this issue on the new site. See the comment by Eoghan O'Brien on this page:
This works on the legacy site:
User account in question on the legacy site: