I received a message that reads "The connection to the KakaoTalk server has been terminated because your phone number is being used on another device," and I am unable to run the app.

If you receive that message without having changed your device, then your Kakao account's email address or password may have been compromised. Please immediately verify your phone number, re-register to KakaoTalk, change your Kakao account password, then log in to KakaoTalk.  

We recommend you use a new and unique password and continue to change your password frequently.