There is nothing fatal about this. Servers will continue to work.
What’s happening is that one of FiveM’s providers, in this case who hosts their Nucleus service, made changes right before Christmas which failed. Due to holidays their provider has not yet solved the problem.
This is a problem completely out of the hands of FiveM. Since its not fatal, you can safely ignore the warnings. Once Nucleus is back up it will reconnect automatically.
FiveM have disabled Nucleus for clients so that players have no impact when connecting through the server list.