The eM Client error messages do not specify which email account is having the problem which means you have to trail through the logs looking for the information. Using eM Client pro version 7.1.33101.0. Have checked for updates but says none available.
If you click on the Log tab, clear the logs then do a Refresh from the main screen, that might make it easier to find. As a Pro License user, you may want to open a support ticket if you are not able to solve this yourself.
7.1.33101 is the latest version available at http://www.emclient.com/release-history.