pidgeon92, when I had the "Error" message a few months ago, I reinstalled Reader Library using these instructions. I am not sure how you did the reinstall, but there are some extra folders that you have to delete, or it won't cleanly reinstall and work properly.
https://www.mobileread.com/forums/showthread.php?t=78921
Then I reformatted my 950. This step might have been overkill. The process was a total pain. It required reloading Reader Library with all of my books and rebuilding my collections. Same with the reformat since you have to then reload all of your books, pictures, music, etc. However, it fixed the problem for me. Very frustrating and time-consuming.
I have an older MBP with Leopard. Everything was fine with Reader Library until something got corrupted. Your situation may be different if your iMac has never recognized your 950.