error opening Excel 2010 file exported from web app

I used to have no problem exporting to Excel from a particular web app, but in June was suddenly unable to export and began to receive the following error (attached) when Excel 2010 opens:

The file you are trying to open, '<filename>.xls' is in a different format than specified by the file extension. Verify that the file is not corrupted and is from a trusted source before opening the file. Do you want to open the file now?

The options I have are Yes, No, and Help

When I click Yes, the file opens but is blank. I have tried the save option (saving as xls, txt, csv, etc.) the app gives me before Excel opens, but the file is still blank. HOWEVER, if I reduce my query parameters to get less results returned, I am able to export the file and open with Excel (and can even save instead of opening in Excel). I have cleared my browser cache, tried different versions of the same browser, and have even tried a different browser.

This is extremely frustrating for me because it is causing me to do multiple exports for different periods of time to get all the data I need, then I have to mesh it all together, remove duplicates, etc.

I have researched this particular error and feel strongly that it is not a registry or trust center issue because I can export successfully smaller files from this very same web app.

I have submitted a bug with the web app's support desk back in June but they don't have a solution and really don't even appear to be working on the problem. Maybe they think that if they ignore it long enough it will go away.

Please help!

the file you are trying to open is in a different format than specified by the file extension.png

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