Error 9997 Can't open OpenEXR files

It's the length of the file path, reducing the length of the path will fix this. Running it on Dropbox/hard drive/network drive will still work. Not sure the exact max length AE will read, but I'd try reducing the length of your folders and file names

The same error occurred when exporting the EXR. There is another commonality, and that is that I was also managing my project files in Dropbox.

I found that when I exported the EXR on Dropbox, the error occurred quite often.

The solution was to export to local storage that is not Dropbox, and so far no errors have occurred.

By the way, I have not seen any errors except for exporting EXR.
For example, exporting Tiffs and pngs is not a problem.
No error occurs when reading EXR files on Dropbox.

The error occurs only when exporting EXR to Dropbox.

ToolfarmJP is correct this is a file permission issue. It looks like you are using dropbox on an external drive. I ha s spoken to Adobe at great lebgth about this and they unfortunately dont support external hardrives. I would reccomend resetting the file permissions on the drive although that didnt work for me. If you copy the files to your desktop they sjould work, if you need them on the external drive then I would copy them back on and it should the problem it. Hopefully Adobe will start supporting external drives but as of a couple of months ago they ahve no intention to. 

I guess two possible causes. One is the permission of the folder. You check everyone can open it. The other is the exr file is corrupted. You check the file can be opened by other applications like Photoshop.