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What version of the java.exe program is executed when the error happens?

What version of java is on the machine where it fails?

Try openning a command prompt and issuing the java command:
java -jar client.jar

Then copy the command prompt console with the error message.
On Windows: To copy the contents of the command...

Is your class name: client.class or is it client? Client.class looks like a filename, not a class name..
If it were named client.class then the name of the class file would be: class.class and it...