ColdFusion 18

If Dave's answer doesn't help, I'll say that it's not clear that your problem is about excel.

First, when you say you get the error "after uploading", do you mean that an error appears on the the page to which you submit a form that does such an upload? Are you sure it's not failing to upload ANY file? Even if your page can "only accept excel files", you may want to try creating a test page to accept other files. What happens?

In any case, I don't see where "the proxy server" would come into play. If you remain stumped, tell us about where the client is that's doing this uploading? Are they on a mobile device? Or maybe connected to their network using their mobile phone to provide their internet connection? Any clue may be helpful.

Did you use MS Office installed on the server with an earlier version of CF?

Dave Watts, Eidolon LLC