Strange, I am having the same issue
'Sorry, it seemed like there was an error during request.'
The individual agent works just fine, but when attached to Meta, it throws this error message in Integration -> Web Demo.
** BTW, this Dialogflow from Google seems so buggy. It is requires to refresh screen to show the intents, also, missing the option to Move/Copy the Intents in sub-tree form. This is basic (i understand there is CX)