I've definitely heard of this error message before. The inventory template is incredibly sensitive. Nothing in the A column can be customized, the alphanumeric ID can't be altered in any way. 

Have you tried downloading a fresh template, and then copying and pasting your content from the previous template? Most of the time this does the trick. 

PS) If my answer solved your problem, would you mind marking it as "Best Answer"? That'll help the rest of the community find what they're looking for. If not, just write back, and I'll continue to help. Thanks!