Hi there,
Thanks for your bug report! I've just released Hero Lab V8.2a, which should fix this issue - could you please grab it and make sure it does? (If it doesn't work, you can revert back to HL 8.1.)
The problem was occurring when you imported a file from the encounter library, saved the file, then reloaded the file - I added some error checks and inadvertently triggered an error in this situation.
Sorry for the inconvenience!