Sage 50 Canada

This past week, all of sudden, I've been having the following issue when I hit "Post" on an account reconciliation: What is this? Thanks, Kristine

I just hit "Post" on an account reconciliation, and Sage 50 up and quit and gave me the following error message: Does anyone know what this means, perchance? How do I avoid this happening in the future? For what it's worth, I have since started…