Note: a member can legally become an ex-member, if he/she or the Admin deletes the profile of said user.
Note: doing ANY upgrade, it is very important to change the install date in the Admin Center > Forum Settings to a date and time "before" the older forums oldest member registration.  If not done, Search will not function and the possibility of causing ex-members being generated, when old members/messages are moved to the new forum version.
 Be sure to run the Maintenance functions (from top to bottom) after the old boards/members/messages have been moved to the new forum (after date/time has been corrected).

Whether it be the Admin or a User .. suddenly becomes an Ex-Member ..
 
Usually the problem is a Server qlitch, that some how changed the Time and/or Date Which reflects in the Admin Center > Forum Settings.
 
This Date/Time of the Forum Installation, MUST be "before" any member has registered.  Check the Members list for the Oldest Registered Member and then Set the Date/Time of the Forum Installation prior (before) that date.  eg.  Admin registered 12/10/2001 ... Forum Installation Date should be 12/09/2001 ... to be safe.
 
Recount/Rebuild Members in the Admin Center > Maintenance.

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Edited:


For any member, the progression of dates should be:

Forum Start Date --> Member Registration Date --> Date of Member's first post.

Member Registration Date is editable in the user's Profile in the Admin Edits section.