I've tested again and it works on UI.X 2 (not dark, but templates should be the same). Maybe your PAGE_CONTAINER template is customized.
Check template modifications for this add-on and check this:
Click the numbers on the right side and check if UI.X dark has number 1:
It seems like the addon works with UI.X and the addon LinkedAccounts when they are seperated, but if I am using both UI.X and LinkedAccounts at the same time, it does not work. Could you have a look at this?
I just tested with UI.X style and Linked Accounts add-on does not work with UI.X even when I disabled my add-on. So, that is not an issue with this add-on. I suggest you to contact author of Linked Accounts add-on.