I can't believe I'm the only user who needs to access pre-2018 files, and did not have the foresight to go back while I did have access and re-save all the older files with the .idml extension.
Now, if I need to open a .indd file from, say, 2015, the latest and greatest version of the program throws me an error message that "Cannot open the file "xxxxx.indd" Adobe InDesign may not support the file format, a plug-in that supports the file format may be missing, or the file may be open in another application."
I did try one trick. I changed the extension manually in the Mac OS finder window of an archived 2015 file, from .indd to .idml and I was able to open that version.
So is this a workaround for this problem? Seems very duct-tapey and paper-clippy for a problem involving older files. But, who knows.
I never have problems opening up pre-2018 InDesign files. Here's one from 2013 opening as an INDD file. The only problems are fonts not available or a Type 1 font which snuck into the file:
Can you tell us: Please tell us what operating system (exactly) you're running. Please tell us (exactly) what InDesign version you're using. I really have never had that happen.
Here's a file from 2007:
>> I did try one trick. I changed the extension manually in the Mac OS finder window of an archived 2015 file, from .indd to .idml and I was able to open that version.
This actually sounds like sombody previously changed .idml extensions to .indd hoping to make that work...
Perhaps it's the plug-in aspect of the error. Did you use any sort of plug-in while working with your earlier files??
Renaming .indd to .idml should not work, ever -- file format is completely different and a renamed file should not be recognized as a valid .idml.