I hate that error message in Excel. When I get it it's usually because I've tried to do something funky with the XML (like passing an XML string into Excel using PRETEXT or POSTTEXT -- really not happy with that one) or doing something with styles or formulas that Excel didn't like. I used to get it all the time when I had a title statement in the code -- before the tagset allowed embedded titles.
I don't have a good way to tell what exactly caused the error in a particular instance, but I do know that when I work with ExcelXP, I keep making numbered copies of my programs. When I get the error, I fall back to the last known program that Excel opened successfully and then ask myself what changed between the working program and the program that Excel gave the error with.
I tried looking down in that error file that Excel logs, but generally don't find it useful.
ps...sometimes, the updated tagset for ExcelXP fixes things.