Sometimes this is caused, not by an invalid character in the file but in the object's title in Tridion Docs. If you open the properties, copy the text out and paste in  a text editor, ensure that there are no leading spaces in the object title, and copy the text back into the object title field, that will often clear the issue. This was especially the case with publications in Reach (the older review environment).

We've had this happen, but unfortunately I can't find anything in my email history about it. Probably because everyone sends me screen shots instead of actual error message text.

My best guess is that you have UTF-16 encoded content that isn't being recognized as that. Could be a UTF-16 file that lost its BOM and/or <?xml ...?> declaration. Or a UTF-16 string embedded in data where UTF-8 or something else is expected.

How this happened due to a computer crash, I have no idea. Maybe the file was under edit and got corrupted?

Thanks Bill, that was a good idea. I did check the XML codes, but they appeared flawless. So looking at the metadata would be useful. I'll keep this in mind if this ever happens again.

Thanks Jay! Yes indeed, I think the damage was done because the file was checked out when my computer decided to go "blue" on me. In this case the UTF codes were good. I compared the codes in the "bad" file with the codes in a "good" file, and since there are not many of them in the top of the file I did establish that everything - including the UTF - were ok.