Received an item upgrade in raid. After raid went to have item enchanted and gemmed. Got the SP enchant on the hands then could not open the gem socket page with shift right click. Thought maybe lagging or glitching so logged out and back in. Still did not work. Logged out for the night. Subsequent days later have discovered that the shift right click on that toon and several others does not work, yet still works fine on some of my other toons.
Character - Geniar
Realm - Ashkandi - US
I seem to remembering having this issue in TBC. It had something to do with an addon.
I was going to direct you to a post I made long ago helping someone who was unable to split a stack of items (shift-click on the stack didn’t work) because “SOCKETITEM” is a modified click just like “SPLITSTACK” is… but /dump GetModifiedClick("SOCKETITEM") returns an empty result for me yet I can still open the socket window, so I am not sure the directions I give in that thread would help you (not sure if Blizzard is doing something different or if I have some addon playing middle man and changing the behavior to not use the modified click settings).
I guess it doesn’t hurt to try though. 
I tried using that and keep getting a nil value. My split stack, socket item, item link to chat are all not working, on mulitple toons yet still work fine on others.