Hi Timo, can you ensure you're using a supported JAVA platform (https://confluence.atlassian.com/display/JIRA/Supported+Platforms). The supported JAVA version for JIRA 6.1.7 is JDK 1.7, be sure you're not using JDK 1.6.
If you have made these JVM change and you're still having problems with loading system plugins, you may want to try the following:
If you're still having problems after this you may want to make sure your server meets JIRA requirements as in https://confluence.atlassian.com/display/JIRA/JIRA+Requirements.
Also, ensure there is no Antivirus scanning your JIRA home or installation directories.
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