Sorry to hear you are having issues with the install.
The file is accessible to me and install works for me so I'm wondering if you might have been caught in some shorttimed outage of the file server or if you might have some custom proxy settings ?
Thanks for your reply Max,
My initial thought was that this was an internet problem so I kept trying the install however it always fails at http://download.jboss.org/jbosstools/static/mars/development/updates/core/4.3.0.Beta1/features/org.jboss.ide.eclipse.as.server.rse.integration.feature_3.1.0.Beta1-v20150612-2158-B25.jar.
I have been able to download the file using the url. It must be an installation script problem.
I have been able to successfully complete the install from http://download.jboss.org/jbosstools/updates/development/mars, selecting the Abridged JBoss Tools and removing JBoss Tools Maven Packaging Configurator, JBoss AS Tools and JBoss Central Community all which include the JBossAS Tools Server / Remote Systems Integration feature.
I initially tried installing from the market place but it had failed as well.