In particular, harvest-fnids has as maximum number of allowed fnids. If there are too many, it purges any fnids that are older than their expiration time, and the oldest 10%.
Thus, the more fnids created (i.e. the more users), the sooner fnids will get harvested and you'll get the expired error.
Comments
My (unofficial) documentation of the Arc web server may help understand this: http://files.arcfn.com/doc/srv.html
In particular, harvest-fnids has as maximum number of allowed fnids. If there are too many, it purges any fnids that are older than their expiration time, and the oldest 10%.
Thus, the more fnids created (i.e. the more users), the sooner fnids will get harvested and you'll get the expired error.