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Download and install Okapi Longhorn as shown here: http://okapiframework.org/wiki/index.php?title=Longhorn#Download_and_Installation.
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To solve this: Create a config file with the name "okapi-longhorn-configuration.xml" in the tomcat user directory.
Strangely under Under Ubuntu 18.04 and Tomcat9 the tomcat user directory is this may be the Linux root directory, meaning "/". To find out if this is the case in your Linux distro, you can look up the user directory for the user "tomcat" or "tomcat9" in /etc/passwd.
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Translate5 Configuration of the Okapi Plug-In
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