Your profile indicates V10.5 so is it safe to work on the basis that that is what you are using/this problem relates to? I have to ask as it's ia not always that simple!
It sounds like the Export Options could be set to use an old version of Excel fiel formats. That would seem odd if you have had previous successes though.
Certainly the first thing to check and eliminate. If that's not it .... hmm. Not sure but I'm confident that someone will come up with further suggestions.
Let us know.