Maybe you don't double-click on the connection to select it. In "Existing connections", you need to double-click on a connection, and then click "Clear" and "Accept". Then you need to double-click the next connection. If you just click on other connection, it's not selected.
For example, see attached pic 1, I have two connections Panel1.height and Panel1.activate. I select "height" and clear it. See pic 2, height is deleted. But if I click on "activate", it is not selected. The connection is still "Panel1.height". Then if I "Accept", it will try to create a new connection because the connection to Panel1.height doesn't exist. WIth the empty parameter, it will return the "expression cannot be empty" error.
So you need to double-click on "activate" to select it and clear it.
Have a try.
Ryan Shi
National Instruments