Hi there, Hoppers!
This is a general message for multiple errors at the exchanges and happens when, for example Kraken returns a 504 Cloudflare page. The page returned is HTML, but we expect JSON to be returned from the API. Instead of saving the complete HTML response received from the exchange, we save this message in the log. You will also see what the hopper was trying to do, because you will see another error message. For example: Could not get balances. This means that when trying to get the balances, the HTML error page was returned and therefore the "get balances" task failed.
This error happens a lot with Kraken hoppers, but also with Bittrex and sometimes with Poloniex and Coinbase PRO. You do not have to do anything when this occurs. The hopper will keep on trying to reach the API and continue its work.