I'm glad to see you were able to resolve your issue. Thanks for posting your solution so that others experiencing the same thing can easily reference this.
Since the Microsoft Q&A community has a policy that the question author cannot accept their own answer, they can only accept answers by others, I'll repost your solution in case you'd like to Accept the answer.
Issue: You observe error 'ajaxExtended call failed' while saving the fallback URI in your connector in designer mode.
Resolve: You found that while referring to output from previous block in logicapp, reference was mis-specified. After correcting the reference, the issue is resolved.
To answer your previous comment : As per your screenshot you are using the standard logic app so it doesn't have the preview designer as it is only available for the consumption logic app and we do see some issue for some of the connector and the suggest was to leverage the fallback URL to troubleshoot the issue whether you are still getting the same error or not.
In case if someone observes the same behavior in case of standard logic app the suggestion would be verify the input/configuration on your action. If it still didn't help, then you can collect fiddler or F12 traces to verify what is the response from the backend service for the actual error as in some cases it may give ajaxExtended call failed error when there is issue/error with the request/configuration on your action. If you still face the issue then you can always create a new QnA post so we can assist you further.