When a scheduler does a callback and the amount of resources change (see Forum 643 (bug)) the amount of tree nodes should be updated as well. If this doesn't happen the 'tree array' can go out of range.
Yes, I should probably fire an exception when you modify the Resources collection but don't call full Update. I don't like the idea of overriding the Update mode automatically. Maybe there could be a fourth update mode: "Auto" that would detect the resources collection change.
I'm adding it to the 5.3 release checklist. Thanks for reporting the problem!
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DayPilot