Without knowing anything about what you changed, I'd guess something is wrong with your unit IDs (so it should be one of the country.inc files in the scenario folder). You either assigned a type/id pair twice, or have a division/ship that belongs to a corps/fleet with another "type". Just searching for all files containing "31600" in your current scenario should turn up likely candidates (later Windows versions do this adequately, but Notepad++ does it more reliably, and is better anyway for editing game files).
EDIT: A missing '{' or '}' is also a likely cause for cryptic error messages or surprising freezes and crashes.