In this situation, its most likley caused because of Multicraft's issue with 1.7.x jars.
Note: It only happens if you are using a JAR file that doesn't match anything in our JAR Selection menu.
To prevent this from happening you can either:
Step 1: Select one of the JAR files from the JAR selection menu and run that.
Step 2: If you are running a custom JAR rename it to custom.jar and select "Special: custom.jar" from the drop down menu
When you do either one above don't forget to click "Save." Then yon click "Stop" wait for the server to stop and start it again.
If any information you see here is outdated or you are still expereining issues, please let us know creating a support ticket. Thank you.