Twilio Dialogflow cx one-click-integration fails with unspecified error, worked before

@edit Through the browser developer console I was able to extract an actual error message:

Invalid AvailableAddOnSid provided

But what does that actually mean. I find nothing on the web and in fact have no Twillio Add-Ons installed.

Yesterday I successfully integrated Dialogflow and Twilio per one-click-integration.

Today I wanted to do the same with another agent but always got an unspecified error when completing the last step in twilio. See screenshot of error in popup of last step in integration:

I did not change anything. What can it be and why isn't the error more specified?