[Zope] [Observation] "None object has no method load"

For the archives: Today, I used a Zope 2.3.3 to extend an application. Suddenly, Zope reported "Attribute error, getUserByName". I check the log file and found "Could not load lots of "<Object id>", "None" object has not method "load". Restarting Zope let the problem disappear. I am unaware, that I did any special things before the problem occured. Dieter