I add tracks to the queue by right-clicking them and then clicking the "add to queue" option.
The output of the "rhythmbox -D alttoolbar" command is:

(23:48:33) [0x556d9726e410] [AltToolbarHeaderBar.move_col] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_type.py:691: Ort
(23:48:33) [0x556d9726e410] [AltToolbarHeaderBar.move_col] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_type.py:691: 11
(23:48:33) [0x556d9726e410] [AltToolbarHeaderBar.move_col] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_type.py:691:  
(23:48:33) [0x556d9726e410] [AltToolbarHeaderBar.move_col] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_type.py:691: Letzte Wiedergabe
(23:48:33) [0x556d9726e410] [AltToolbarHeaderBar.move_col] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_type.py:691: 12
(23:48:33) [0x556d9726e410] [AltToolbarHeaderBar.move_col] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_type.py:691:  
(23:48:33) [0x556d9726e410] [AltToolbarHeaderBar.move_col] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_type.py:691: Hinzufügedatum
(23:48:33) [0x556d9726e410] [AltToolbarHeaderBar.move_col] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_type.py:691: 13
(23:48:33) [0x556d9726e410] [AltToolbarHeaderBar.move_col] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_type.py:691:  
(23:48:33) [0x556d9726e410] [AltToolbarHeaderBar.move_col] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_type.py:691: BPM
(23:48:33) [0x556d9726e410] [AltToolbarHeaderBar.move_col] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_type.py:691: 14
(23:48:33) [0x556d9726e410] [AltToolbarHeaderBar.move_col] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_type.py:691:  
(23:48:33) [0x556d9726e410] [AltToolbarHeaderBar.move_col] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_type.py:691: Titel
(23:48:34) [0x556d9726e410] [AltToolbarHeaderBar._save_entryview_cols] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_type.py:897: entryview column changed
(23:48:34) [0x556d9726e410] [AltToolbarHeaderBar._save_entryview_cols] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_type.py:898: <__gi__.RBPlayQueueSource object at 0x7f968c04bc60 (RBPlayQueueSource at 0x556d975502d0)>
(23:48:34) [0x556d9726e410] [AltToolbarHeaderBar._save_entryview_cols] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_type.py:937: 'Genre','Interpret','Komponist','Album','Jahr','Länge','Qualität','Bewertung','Wiedergaben','Kommentar','Ort','Letzte Wiedergabe','Hinzufügedatum','BPM','Titel','Titel'
(23:48:34) [0x556d9726e410] [AltToolbarSidebar.delayed] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_sidebar.py:1021: {1: True, 2: False, 0: False, 3: False}
(23:48:36) [0x556d9726e410] [AltToolbarSidebar._set_text] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_sidebar.py:1117: Lokale Sammlung
(23:48:36) [0x556d9726e410] [AltToolbarSidebar._set_text] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_sidebar.py:1117: Lokale Sammlung
(23:48:36) [0x556d9726e410] [AltStandardOnlineController.valid_source] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_controller.py:1358: RBPodcastMainSource
(23:48:36) [0x556d9726e410] [AltStandardLocalController.valid_source] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_controller.py:1412: RBPodcastMainSource
(23:48:36) [0x556d9726e410] [AltErrorsController.valid_source] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_controller.py:1013: RBPodcastMainSource
(23:48:36) [0x556d9726e410] [AltStandardOnlineController.valid_source] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_controller.py:1358: RBPodcastMainSource
(23:48:36) [0x556d9726e410] [AltStandardLocalController.valid_source] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_controller.py:1412: RBPodcastMainSource
(23:48:36) [0x556d9726e410] [AltErrorsController.valid_source] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_controller.py:1013: RBPodcastMainSource
(23:48:36) [0x556d9726e410] [AltToolbarSidebar._set_text] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_sidebar.py:1117: Onlinequellen
(23:48:36) [0x556d9726e410] [AltToolbarSidebar._set_text] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_sidebar.py:1117: Onlinequellen
(23:48:36) [0x556d9726e410] [AltToolbarSidebar._set_text] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_sidebar.py:1117: Wiedergabelisten
(23:48:36) [0x556d9726e410] [AltToolbarSidebar._set_text] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_sidebar.py:1117: Wiedergabelisten
(23:48:36) [0x556d9726e410] [AltToolbarSidebar._model_page_changed] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_sidebar.py:667: Warteschlange (2)
(23:48:36) [0x556d9726e410] [AltToolbarSidebar._model_page_changed] /usr/lib/rhythmbox/plugins/alternative-toolbar/alttoolbar_sidebar.py:668: 0:0

ERROR: Caught a segmentation fault while loading plugin file:
/usr/lib/x86_64-linux-gnu/gstreamer-1.0/libgsttypefindfunctions.so

Please either:
- remove it and restart.
- run with --gst-disable-segtrap --gst-disable-registry-fork and debug.