The plugin for plagiarism detection uses an wsdl file for accessing JPlag. (I've only checked version 2.2 but) The wsdl file contained in the plugin is outdated and does not work anymore. Please update with the current version available on [1]. Also the documentation [3] and the readme refers to the old web pages (uni-karlsruhe.de is now know as kit.edu); the new address is [2].

Regards
Mathias

[1] https://jplag.ipd.kit.edu/JPlagService/service
[2] https://jplag.ipd.kit.edu/
[3] https://docs.moodle.org/27/en/Programming_Code_Plagiarism_Plugin