PureBasic Forums - English

Return: If the manual doesn't mention for exit loop it, then don't use it, even if it works because it doesn't say that it will always work in all cases.
The official and therefore safe way out of a loop is: Break
Break : Continue

Thanks, Demivec!

Just to finish off, after a great deal of investigation, the problem turned out to be a faulty network switch. Replacing the switch solved the problem. The switch was causing brief random network disconnect. When that happened, Windows shut down the exe without generating an error. This explains the random nature, the lack of error messages with OnError active, and why changing or fixing the software made no difference. It was difficult to track down and involved considerable patience on the part of the school library and technical support staff. Thank you everyone here for the suggestions.

@DeanH: Phew, glad that's over! Don't you just hate chasing non-existent bugs in your code? LOL.