Thanks for the information that you sent us.
I can see that the connection is working well when used.
Regarding the errors that you are seeing in the designer, you can do the following
- Run a fiddler and get us the trace
- Look at the F12 Netowrk traffic and send us the failing call.
- To do this, delete the trigger card.
- Launch the F12 diagnostics and record "Network" calls
- Add the trigger and select the sharepoint site.
- Proceed to click the "Down Arrow" in the List name, at which point we should have enough captures indicating the errors.
Please get back to us with this information and we should able to help you.