Hello Stefan,
you can have up to 4 active debug sessions per project in parallel. If you get the above message try to open the debug view (e.g. via switch to debug perspective) and check if there are still old debug sessions remaining. You can close them via the terminate button from the toolbar.
Depending on your NetWeaver release it may happen that old debug sessions remain if you close the tabs with embedded GUIs without explicitly terminating the debugged process or the debugger before closing the tab.
Best regards,
Armin