[kens]

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.