The signature is that of a previous version.
A possible reason for the error is that the Server in use is version 7.0.x or earlier and the adapter interface jar is for a later Server version.
Obviously, we assume that some change has happened involving the classpath.
For instance, you may have downloaded the latest version of the adapter interface jar from maven or from github and you have placed it before the older at Lightstreamer\lib\ls-adapter-interface.jar.