Thanks to all tester. Now we can say: it's an Infopop Job I will report it as a bug. It must have something to do with the database itself. The error only appears in one forum (ubb14) and no where else. In this forum I have many deleted users (inactiviy purge). The forum itself is an imported one from a UBB.classic.
/edit: I see, it's already reported and I will do the workaround.
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I will report it as a bug. It must have something to do with the database itself. The error only appears in one forum (ubb14) and no where else. In this forum I have many deleted users (inactiviy purge). The forum itself is an imported one from a UBB.classic. 
