Exception: PKIX path building failed!

[ubidragon]

For more than week I can't log into game. In school, game working, but in home After start client I have message: Could not connect to server.
I trying on client log on one more time and I have another error: Exception: PKIX path building failed: sun.security.SunCertPathBuilderException: unable to find valid.... (something).
I have try to reinstall java, and enabling into settings, but it still don't working. On Internet java test sites, the java is working correctly.