When a customer imports data into Cloud from a Server instance, or when a Cloud instance is migrated to a new url any installed Connect add-ons break.
This is because the clientKey is generated from the OAuth consumer key, which is seeded from the instance baseUrl. If the baseUrl changes, already installed add-ons may stop working.
Ask the vendor to update their registry and update the baseUrl/clientKey to the new values.
Reinstall the add-on. Note that this may cause data loss depending on the add-on, if the remote service stores data which is keyed off the old clientKey/baseUrl combo. Speak to the add-on vendor before doing this.
Implement a contract for Atlassian Connect to allow vendors to be notified when an instance has been migrated so that any data can be appropriately migrated.