Actually, I just pulled the header off the file, and now it says the error is at line 824 and simply says the error is "k" Not sure what to think now.
Are you saving to a network drive or a local drive? I have noticed corrupt notebooks are very rare when saving to a local drive (at least under Windows).
By the way, take a look at https://resources.wolframcloud.com/FunctionRepository/resources/SalvageNotebook/
Thanks for this! I wasn't even aware of this option!