I have the exact same problem using QB's Accountant Desktop 2019 and Excel 2016. I create reports and cannot reopen them.  The error is the same "Only ISW 2009 files can be imported..."  I tried going through the recommended "fixes" without any success - still cannot open anything I've created.  What ended up working for you?  Any help is appreciated! 

In case it's applicable - I have several full versions of QB's (from 2010 onward) loaded on my machine due to many of our clients' using different version years.