Error
This is a standby Vault node but can’t communicate with the active node via request forwarding. Sign in at the active node to use the Vault UI.
Why are we facing this error all of sudden and application is throwing that error.
The error is saying that your web browser is communicating with a Vault node that is in standby mode, because it can see in the configured storage backend that another node is active - and yet, when it tries to speak to that active node at the URL it has written into storage, that communication isn’t working.
It is impossible to be more helpful as you have given no information about your Vault deployment.
A description of your HA setup, including loadbalancers, and the contents of the Vault server configuration file (with notes on how it differs between each node) would be a good starting point.