I believe you are adding the Node to email listener as mentioned in the below screenshot,
Once you get error message, Open clipboard -> goto page where the error is seen -> Edit the property which is throwing error and make it empty -> Save the Page and Now in designer studio and try to save the Listener.
In Lower env(Dev,SIT,UAT),we are able to see "Keep messages on server (IMAP)" checkbox in Email Listener data instance. But prodcution env,we are unable to see this checkbox.
There are more than 300 email listeners are there.for all listeners this checkbox is not available. without this checkbox also,we are able to add new nodes to email listeners.But only 3 emaillisteners we are unable add new nodes and getting this error "pykeepmessagesonserver" validation failed.
Note: I am not sure,why this checkbox not there in only prodcution env.
Do you see this issue in development environment too? can you please check and do following:
"With regards to this issue, please check Data-Admin-Connect-EmailListener.pyDefault model rule in your environment. See if this property pyKeepMessagesOnServer is being initialized there. If it is, you can save the model in a custom ruleset and delete that step."