I've never experienced this problem before but for some reason the most basic of expressions are not linking to other values either in the same comp or in other comps.
Take a look. This example just shows a basic pickwhip from one opacity value to another.
I really hope someone can help me figure out what's going on.
Check your layer names and avoid spaces. This is a bug where extra invisible characters are added in such a case and then of course the expressions seem to reference the wrong layer and break down.
But I haven't been typing the expressions, just pick whipping them, which is why I don't understand what's happening.
If you have renamed of your layers will run into this bug. To fix the renaming select the later you want to edit, press the enter key, then press the delete key twice and then type in the new name. This will fix the extra line feed and your expressions will work.