modem frequently disappears (ttyUSB1 timed out 10 consecutive times, marking modem as invalid)

On an uptodate L5 with new modem firmware (but issue existed before already) I face from time to time the modem disappearing; journalctlshows:

Apr 25 09:08:15 pureos sudo[23493]:   purism : TTY=pts/2 ; PWD=/home/purism ; USER=root ; COMMAND=/usr/bin/apt update
Apr 25 09:08:15 pureos sudo[23493]: pam_unix(sudo:session): session opened for user root(uid=0) by (uid=1000)
Apr 25 09:09:00 pureos sudo[23493]: pam_unix(sudo:session): session closed for user root
Apr 25 09:09:59 pureos ModemManager[5761]: <warn>  [modem4] port ttyUSB1 timed out 8 consecutive times
Apr 25 09:10:04 pureos ModemManager[5761]: <warn>  [modem4] port ttyUSB1 timed out 9 consecutive times
Apr 25 09:10:07 pureos ModemManager[5761]: <warn>  [modem4/bearer9] connection attempt #1 failed: Serial command timed out
Apr 25 09:10:07 pureos ModemManager[5761]: <info>  [modem4] state changed (connecting -> registered)
Apr 25 09:10:07 pureos ModemManager[5761]: <info>  [modem4/bearer9] connection #1 finished: duration 0s, tx: 0 bytes, rx: 0 bytes
Apr 25 09:10:07 pureos ModemManager[5761]: <error> [modem4] port ttyUSB1 timed out 10 consecutive times, marking modem as invalid
Apr 25 09:10:07 pureos mmsdtng[867]: ../plugins/modemmanager.c:modem_state_changed_cb() State Change: Old State: 10 New State: 8, Reason: 1
Apr 25 09:10:07 pureos mmsdtng[867]: ../plugins/modemmanager.c:mmsd_mm_get_modem_state() MM_MODEM_GOOD_STATE: 8
Apr 25 09:10:07 pureos mmsdtng[867]: ../plugins/modemmanager.c:mmsd_mm_state() MMSD_MM_STATE_READY
Apr 25 09:10:07 pureos mmsdtng[867]: ../plugins/modemmanager.c:mmsd_mm_state() Setting Bearer Handler
Apr 25 09:10:07 pureos mmsdtng[867]: ../src/service.c:mms_service_set_bearer_handler() service 0xaaaad27f8370 handler 0xaaaab6475a20
Apr 25 09:10:07 pureos NetworkManager[567]: <info>  [1682406607.6709] modem["ttyUSB1"]: modem state changed, 'connecting' --> 'registered' (reason: user-requested)
Apr 25 09:10:07 pureos NetworkManager[567]: <warn>  [1682406607.6712] modem-broadband[ttyUSB1]: failed to connect modem: Serial command timed out
Apr 25 09:10:07 pureos NetworkManager[567]: <info>  [1682406607.6714] device (ttyUSB1): state change: prepare -> failed (reason 'unknown', sys-iface-state: 'managed')
Apr 25 09:10:07 pureos mmsdtng[867]: ../plugins/modemmanager.c:mmsd_modem_unavailable() Disabling Bearer Handler
Apr 25 09:10:07 pureos mmsdtng[867]: ../src/service.c:mms_service_set_bearer_handler() service 0xaaaad27f8370 handler (nil)
Apr 25 09:10:07 pureos mmsdtng[867]: ../plugins/modemmanager.c:mmsd_disconnect_from_sms_wap() Stopping watching SMS WAPs
Apr 25 09:10:07 pureos mmsdtng[867]: ../plugins/modemmanager.c:mmsd_mm_state() Modem vanished, Disabling plugin
Apr 25 09:10:07 pureos mmsdtng[867]: ../plugins/modemmanager.c:mmsd_mm_state() MMSD_MM_STATE_NO_MODEM
Apr 25 09:10:07 pureos mmsdtng[867]: ../plugins/modemmanager.c:cb_object_removed() Modem removed: /org/freedesktop/ModemManager1/Modem/4
Apr 25 09:10:07 pureos NetworkManager[567]: <warn>  [1682406607.6798] device (ttyUSB1): Activation: failed for connection 'Fonic'
Apr 25 09:10:07 pureos NetworkManager[567]: <info>  [1682406607.6823] device (ttyUSB1): state change: failed -> disconnected (reason 'none', sys-iface-state: 'managed')
Apr 25 09:10:07 pureos NetworkManager[567]: <info>  [1682406607.6897] device (ttyUSB1): state change: disconnected -> unmanaged (reason 'removed', sys-iface-state: 'removed')
Apr 25 09:10:32 pureos sm.puri.OSK0.desktop[1285]: Debug: Tried file "/home/purism/.local/share/squeekboard/keyboards/terminal/us.yaml", but it's missing: No such file or directory (os error 2)
Apr 25 09:10:32 pureos sm.puri.OSK0.desktop[1285]: Info: Loaded layout Resource: terminal/us
Apr 25 09:10:34 pureos dbus-daemon[875]: [session uid=1000 pid=875] Activating service name='org.gnome.DejaDup' requested by ':1.29' (uid=1000 pid=1064 comm="/usr/libexec/phosh ")
Apr 25 09:10:34 pureos sm.puri.OSK0.desktop[1285]: Debug: Tried file "/home/purism/.local/share/squeekboard/keyboards/terminal/us.yaml", but it's missing: No such file or directory (os error 2)
Apr 25 09:10:34 pureos sm.puri.OSK0.desktop[1285]: Info: Loaded layout Resource: terminal/us
Apr 25 09:10:35 pureos dbus-daemon[875]: [session uid=1000 pid=875] Successfully activated service 'org.gnome.DejaDup'
Apr 25 09:10:35 pureos sm.puri.OSK0.desktop[1285]: Debug: Tried file "/home/purism/.local/share/squeekboard/keyboards/terminal/us.yaml", but it's missing: No such file or directory (os error 2)
Apr 25 09:10:35 pureos sm.puri.OSK0.desktop[1285]: Info: Loaded layout Resource: terminal/us
Apr 25 09:10:35 pureos deja-dup[23788]: g_object_set: assertion 'G_IS_OBJECT (object)' failed
Apr 25 09:10:35 pureos sm.puri.OSK0.desktop[1285]: Debug: Tried file "/home/purism/.local/share/squeekboard/keyboards/terminal/us.yaml", but it's missing: No such file or directory (os error 2)
Apr 25 09:10:35 pureos sm.puri.OSK0.desktop[1285]: Info: Loaded layout Resource: terminal/us
Apr 25 09:10:36 pureos sm.puri.OSK0.desktop[1285]: Debug: Tried file "/home/purism/.local/share/squeekboard/keyboards/terminal/us.yaml", but it's missing: No such file or directory (os error 2)
Apr 25 09:10:36 pureos sm.puri.OSK0.desktop[1285]: Info: Loaded layout Resource: terminal/us
Apr 25 09:10:42 pureos wpa_supplicant[628]: wlan0: WPA: Group rekeying completed with cc:ce:1e:7b:f5:b9 [GTK=CCMP]
Apr 25 09:10:42 pureos kernel: redpine_91x: Packet Dropped as Key ID not matched with both current and previous Key ID
Apr 25 09:10:42 pureos kernel: redpine_91x: Packet Dropped as Key ID not matched with both current and previous Key ID
Apr 25 09:10:53 pureos sm.puri.Phosh.desktop[6209]: Missing chrome or resource URL: resource://gre/modules/UpdateListener.jsm
Apr 25 09:10:53 pureos sm.puri.Phosh.desktop[6209]: Missing chrome or resource URL: resource://gre/modules/UpdateListener.sys.mjs
Apr 25 09:10:55 pureos systemd[843]: vte-spawn-d7951a5a-42b3-466c-a4a4-bd3dfdb5f19e.scope: Succeeded.
Apr 25 09:10:55 pureos systemd[843]: vte-spawn-d7951a5a-42b3-466c-a4a4-bd3dfdb5f19e.scope: Consumed 1.064s CPU time.
Apr 25 09:10:55 pureos systemd[843]: vte-spawn-2d3d46c0-02c6-4d5b-878d-927c2d6f2eeb.scope: Succeeded.
Apr 25 09:10:55 pureos systemd[843]: vte-spawn-2d3d46c0-02c6-4d5b-878d-927c2d6f2eeb.scope: Consumed 1min 29.398s CPU time.
Apr 25 09:10:56 pureos dbus-daemon[875]: [session uid=1000 pid=875] Activating via systemd: service name='org.freedesktop.Tracker1' unit='tracker-store.service' requested by ':1.2' (uid=1000 pid=872 comm="/usr/libexec/tracker-miner-fs ")
Apr 25 09:10:56 pureos systemd[843]: Starting Tracker metadata database store and lookup manager...
Apr 25 09:10:56 pureos dbus-daemon[875]: [session uid=1000 pid=875] Successfully activated service 'org.freedesktop.Tracker1'
Apr 25 09:10:56 pureos systemd[843]: Started Tracker metadata database store and lookup manager.
Apr 25 09:11:01 pureos yad[23844]: ../../../../../gdk/x11/gdkwindow-x11.c:5650 drawable is not a native X11 window
Apr 25 09:11:01 pureos phosh[1064]: Could not find application for app-id 'yad'
Apr 25 09:11:01 pureos sm.puri.OSK0.desktop[1285]: Debug: Tried file "/home/purism/.local/share/squeekboard/keyboards/terminal/us.yaml", but it's missing: No such file or directory (os error 2)
Apr 25 09:11:01 pureos sm.puri.OSK0.desktop[1285]: Info: Loaded layout Resource: terminal/us
Apr 25 09:11:03 pureos sm.puri.Phosh.desktop[23839]: /home/purism/Pictures/2023-04-25-091100.png|5|
Apr 25 09:11:09 pureos dbus-daemon[875]: [session uid=1000 pid=875] Activating via systemd: service name='org.freedesktop.Tracker1.Miner.Extract' unit='tracker-extract.service' requested by ':1.2' (uid=1000 pid=872 comm="/usr/libexec/tracker-miner-fs ")