Magento 2: Magento has been switched off because of nonce error
For Magento 2 connections we make use of the internal API that Magento 2 offers. The authentication for that API-connection is done via OAuth. Part of the whole OAuth integration is the calculation of a cryptographic 'nonce' which is a random number (or string) used in the authentication protocol to ensure old communications and authorization can't be re-used by a third party (for attacks or intercepting data for example).
The bottom line is that this nonce has to be unique for each request (in combination with a timestamp) and can't be re-used. This isn't a scenario that occurs often, however, sometimes the nonce is re-used by calls done by Magento. This is suspected to be due to specific server configurations, especially regarding redirects. When this issue does occur, this results in errors and failed tasks.
Because there is not much that can be done from ChannelEngine's side, we automatically disable the Magento 2 plugin and throw a notification. In most cases re-enabling the plugin will be sufficient to fix the problem, but if this issue occurs often please contact your (web)server-administrator because some changes will most likely be needed in the configuration of your webserver. Some tips can be found here (even though the original post is for Magento 1.7, the OAuth procedure still works the same).