The number of events allocated is too small
The "Maximum and Average # Events In Each Receiver" in the dialog "Set Event Attributes" is the number of events for all the ray codes together. If the number of ray codes is large, it may lead to a stop in the modelling when the total number of events allocated is reached.
Increase the "Maximum and Average # Events In Each Receiver" so that all events for all ray codes may be stored in memory.