@Aspargos: works different with JDownloader. You've got to open the JDownloader app with rightclick and there you should find a vmoptions file. Edit it with texteditor and then remove any existing Xmx or Xms line and add -Xmx6g for 6GB of memory.
I can also help via Teamviewer if you like, just contact me via support@jdownloader.org
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JD-Dev & Server-Admin
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