Im having the same problem on Lightroom Classic CC (latest update 2018, MacOS) and just did something that "fixed" it. I read on another post that the user linked the problem to files on an external drive, which is just as I work. I copied the folder containing my ".lrcat" file from the external drive to the desktop, and then proceeded to double click the .lrcat file which opened it on LR. When I clicked over a the photo I was trying to edit previously LR told me it couldnt locate the file, and asked my if wanted to do it. I then made another new folder on my desktop, named it "photo" and dragged the corresponding .rar file from the external drive in to it. I went back LR, "located" the file on this new folder on my desktop and proceeded to the "Photo>Edit in" menu and found "Edit in Photoshop" enabled, which previously I found both greyed out and enabled in different occasions but didn't work regardless, but this time when I clicked it, it all worked as usual, opening the file on PS and saving the edited PS on my same LR collection. Im doing this right now and this is as far as Ive gone, but im guessing Im going to try moving my "lrcat" folders and its corresponding ".rar" files folders from my external drive to the HD on my mac, and then back to the external drive, with a different name or something. Let me know if this works or is of any help! *Update* I did that only with the .lrcat folder and its all back to normal, no need to move all the .rar files. I did all the bunch of stuff described above, but maybe just moving the .lrcat folder back and forth, after deleting the original on the external HD could fix it. It sounds like a file corruption issue.
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