Really strange, there\'s no mistery on installing OpenKM, and the error that appears to you is an UI error starting up, that only occurs when there\'s some compilation error on GWT ( the interface ) that is not possible in the community edition that you\'ve downloaded.
Give me more information:
Operating system
Java version
The starting server.log ( delete it, authenticate and send us the new generated file)
Which browser do you use ?
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OpenKM Team ( be open my friend )
http://www.openkm.com