The letter "J" means, that the photo is part of a "RAW" and "JPEG" pair, and the JPEG file has been selected as the original image file. If the RAW file is selected as the original, you would be seeing a "R" in the corner.
What exactly happens, when you try to edit these photos? Do you get a message about a missing file? Or what is happening?
Try to switch from the JPEG original to the RAW original. Open the image in EDIT mode, if you can, and then use the command "Image > Use RAW as Original". This command is only available in Edit mode.
