ePub validates and opens in everything but ADE?

I’m scratching my head on this. I generated an ePub from InDesign, edited it in Oxygen Author, and it validates. I can open it on all my test devices, but it won’t open on ADE (Adobe Digital Editions). It says there is an error with the file (it won’t tell me what the error is), and it can’t open it. I am able to upload the file to B&N, Kindle, Kobo, Apple, and Google Play, and no problems were detected (and the eBook is on sale already at these places).

I did a little research and some people have noted problems with the new ADE version, so I went back to ADE v4.0 as a test. Still can’t open the file. eBooks I generated before November using the exact same process on my checklist all open on ADE and everywhere else. But not this one file.

If it would at least tell me what the problem is, I could fix it, but all it does is say there’s an error and it can’t open the file. I even checked the file using the online ePub validator from IDPF and it was validated. I downloaded Pagina and checked the file and it was validated with no errors.

Has anyone else been having issues with ePubs on ADE lately? Any ideas on how to discover the problem? I want to upload this to Overdrive, which uses ADE for all the libraries, so I know this is going to be a problem.

Thanks.