Not sure what's going on, but you can check to see if you have more than one version of ACR installed. The file is located here:
C:\Program Files (x86)\Common Files\Adobe\Plug-Ins
I have one of CC and for CS6, but when I tried to open a file in CS6, it defaulted to the newer version of ACR, and would not open, as that version isn't supported by CS6. Have you tried resetting ACR preferences: press ctrl-alt-shift right after you double click to open a file - if you don't get a dialog box asking you if you want to reset your preference, you didn't press the keys fast enough.
I removed all the older versions of Photoshop from the machine. It will take some time to know if that helps as the problem would only occur sporadically.
You apparently didn't read the section of my post where I specifically said I was running an Apple iMac.
Yea, I missed that, I guess.
I think this bug is in the same category as I previously reported: ACR opens NEF without previously applied settings
I have the impression that ACR sometimes either has trouble reading XMP sidecar files, or (worse) writing them. It is very frustrating that nobody at Adobe seems to care.
Agreed on both. ACR seems to have trouble writing to XMP sidecar files and yes Adobe seems to have created this forum so we can all talk to each other an feel like we are doing something and so we don't feel as frustrated that they take our money every month and don't allow any sort of way of formally submitting bug reports.
It just happened again; I had been working on a quite complex image for about an hour. When I thought I was finished I opened it in Photoshop, and discovered that I had made a small mistake. Closed the photo in PS and reopened the NEF in ACR and it came up without any of the edits I made. I tried opening the XMP via the image settings menu; that seemed to work and it now contains the name of the XMP with a tick next to it, but still no edits in the image.
The XMP is 1.6MB (unedited files typically 2KB) so there must be data in it...