To resolve this error, try one or all of the following:
1. Go to Window–>Preferences–>Server–>Runtime Environments and fix the broken path/link for the server
2. Rename the org.eclipse.jst.server.tomcat.core.prefs to org.eclipse.jst.server.tomcat.core.prefs.bak (or you can delete this file).
This file can be found at workspace > .metadata > .plugins > org.eclipse.core.runtime > .settings
3. Rename the org.eclipse.wst.server.core.prefs to org.eclipse.wst.server.core.prefs.bak (or delete the file). This file also can be found at the same location as above