Hi Jerome, thanks for your reply and your work! I am really looking forward for Paperwork 2.0! 
I’ve just freshly rebooted my system and as the first step after running flatpak run work.openpaper.Paperwork I’ve opened the settings. Unfortunately, nothing much happens here as well: The device dropdown names my scanner, but “default source” and “resolution” remain empty. The terminal throws:
INFO paperwork.frontend.util.actions Action: [Open settings dialog]
/app/lib/python3.7/site-packages/paperwork-1.3.1-py3.7.egg/paperwork/frontend/util/__init__.py:163: Warning: g_value_type_transformable: assertion 'src_type' failed
(paperwork:3): Gtk-WARNING **: 18:23:55.005: gtkliststore.c:834: Unable to convert from (null) to gchararray
INFO paperwork.frontend.settingswindow Language name translations loaded
INFO paperwork.frontend.settingswindow Translation: German | Deutsch
INFO paperwork.frontend.settingswindow Translation: English | Englisch
/app/lib/python3.7/site-packages/paperwork-1.3.1-py3.7.egg/paperwork/frontend/settingswindow/__init__.py:881: Warning: unable to set property 'xpad' of type 'guint' from value of type 'gchararray'
INFO paperwork.frontend.util.jobs [Scheduler progress] Starting
INFO paperwork.frontend.util.jobs [Scheduler progress] Started
/usr/lib/python3.7/site-packages/gi/overrides/GLib.py:497: Warning: unable to set property 'xpad' of type 'guint' from value of type 'gchararray'
super(MainLoop, self).run()
INFO paperwork.frontend.settingswindow Looking for scan devices ...
INFO paperwork.paperwork ../subprojects/libinsane/src/bases/sane.c:L212(ensure_sane_is_init): Sane version code: 0x100001B
INFO paperwork.paperwork ../subprojects/libinsane/src/bases/sane.c:L311(lis_sane_list_devices): Sane: list_devices() ...
INFO paperwork.paperwork ../subprojects/libinsane/src/bases/sane.c:L364(lis_sane_list_devices): Sane: list_devices(): 1 devices found
INFO paperwork.frontend.settingswindow Device found: [Brother ADS-2100e] -> [sane:net:127.0.0.1:brother4:bus3;dev2]
INFO paperwork.frontend.settingswindow End of scan for device
INFO paperwork.frontend.settingswindow Got value [['Brother ADS-2100e', 'sane:net:127.0.0.1:brother4:bus3;dev2']]
WARNING paperwork.frontend.util.jobs Job DeviceFinder:0 took 8336ms and is unstoppable ! (maximum allowed: 500ms)
INFO paperwork.frontend.settingswindow Selected scanner: sane:net:127.0.0.1:brother4:bus3;dev2
INFO paperwork.frontend.settingswindow Looking for sources of device [sane:net:127.0.0.1:brother4:bus3;dev2]
INFO paperwork.paperwork ../subprojects/libinsane/src/bases/sane.c:L385(lis_sane_get_device): Sane: get_device(net:127.0.0.1:brother4:bus3;dev2) ...
ERROR paperwork.paperwork ../subprojects/libinsane/src/workarounds/cache.c:L568(cache_get_device): Failed to get_device(sane:net:127.0.0.1:brother4:bus3;dev2): 0x40000003, Invalid value
ERROR paperwork.paperwork ../subprojects/libinsane/src/normalizers/source_nodes.c:L442(lis_sn_get_device): wrapped->get_device() failed: 0x40000003, Invalid value
ERROR paperwork.frontend.util.jobs ===> Job SourceFinder:0 raised an exception: <class 'gi.repository.GLib.GError'>: libinsane-quark: Libinsane get devices error: 0x40000003, Invalid value (4)
ERROR paperwork.frontend.util.jobs 0: /app/lib/python3.7/site-packages/paperwork-1.3.1-py3.7.egg/paperwork/frontend/util/jobs.py: L 180: _run
ERROR paperwork.frontend.util.jobs 1: /app/lib/python3.7/site-packages/paperwork-1.3.1-py3.7.egg/paperwork/frontend/settingswindow/__init__.py: L 171: do
ERROR paperwork.frontend.util.jobs ---> Job SourceFinder:0 was started by:
ERROR paperwork.frontend.util.jobs 0: /app/bin/paperwork: L 11: <module>
ERROR paperwork.frontend.util.jobs 1: /app/lib/python3.7/site-packages/paperwork-1.3.1-py3.7.egg/paperwork/paperwork.py: L 222: main
ERROR paperwork.frontend.util.jobs 2: /app/lib/python3.7/site-packages/paperwork-1.3.1-py3.7.egg/paperwork/paperwork.py: L 210: main
ERROR paperwork.frontend.util.jobs 3: /usr/lib/python3.7/site-packages/gi/overrides/GLib.py: L 497: run
INFO paperwork.frontend.settingswindow Selected device: sane:net:127.0.0.1:brother4:bus3;dev2
ERROR paperwork.frontend.util.jobs 4: /app/lib/python3.7/site-packages/paperwork-1.3.1-py3.7.egg/paperwork/frontend/settingswindow/__init__.py: L 558: _do
INFO paperwork.frontend.settingswindow Selected source:
ERROR paperwork.frontend.util.jobs 5: /app/lib/python3.7/site-packages/paperwork-1.3.1-py3.7.egg/paperwork/frontend/util/jobs.py: L 256: schedule
INFO paperwork.frontend.settingswindow Scanner sources: []
… and nothing more happens. Do you have any more ideas? :-/