There does not seem to be anything relevant in the server NFS logs. I can try harder, I can try to add verbose and check that the logs are actually being produced, if this does not seem to lead us to the problem source:

With "journalctl -f" only, I can get the following messages. A lot of them seem to be related to document portal. I checked the file flatbak/db/documents and it does not seem to exist.

loka 29 19:44:23 koira dbus-daemon[4642]: [session uid=1000 pid=4642] Activating via systemd: service name='org.freedesktop.portal.Documents' unit='xdg-document-portal.service' requested by ':1.196' (uid=1000 pid=14113 comm="/snap/bin/chromium " label="unconfined")
loka 29 19:44:23 koira systemd[4622]: Starting flatpak document portal service...
loka 29 19:44:23 koira xdg-document-portal[14171]: error: Failed to load db: Error opening file /home/XXX/.local/share/flatpak/db/documents: No such file or directory
loka 29 19:44:23 koira systemd[4622]: xdg-document-portal.service: Main process exited, code=exited, status=2/INVALIDARGUMENT
loka 29 19:44:23 koira systemd[4622]: xdg-document-portal.service: Failed with result 'exit-code'.
loka 29 19:44:23 koira systemd[4622]: Failed to start flatpak document portal service.
loka 29 19:44:39 koira xdg-desktop-por[4802]: Failed to get application states: GDBus.Error:org.freedesktop.portal.Error.Failed: Could not get window list: GDBus.Error:org.freedesktop.DBus.Error.AccessDenied: App introspection not allowed
loka 29 19:45:39 koira xdg-desktop-por[4802]: Failed to get application states: GDBus.Error:org.freedesktop.portal.Error.Failed: Could not get window list: GDBus.Error:org.freedesktop.DBus.Error.AccessDenied: App introspection not allowed
loka 29 19:46:23 koira dbus-daemon[4642]: [session uid=1000 pid=4642] Failed to activate service 'org.freedesktop.portal.Documents': timed out (service_start_timeout=120000ms)
loka 29 19:46:24 koira audit[1013]: USER_AVC pid=1013 uid=105 auid=4294967295 ses=4294967295 msg='apparmor="DENIED" operation="dbus_method_call"  bus="system" path="/" interface="org.freedesktop.DBus.ObjectManager" member="GetManagedObjects" mask="send" name="org.bluez" pid=14113 label="snap.chromium.chromium"
                                     exe="/usr/bin/dbus-daemon" sauid=105 hostname=? addr=? terminal=?'
loka 29 19:46:24 koira kernel: audit: type=1107 audit(1572371184.015:58): pid=1013 uid=105 auid=4294967295 ses=4294967295 msg='apparmor="DENIED" operation="dbus_method_call"  bus="system" path="/" interface="org.freedesktop.DBus.ObjectManager" member="GetManagedObjects" mask="send" name="org.bluez" pid=14113 label="snap.chromium.chromium"
                                exe="/usr/bin/dbus-daemon" sauid=105 hostname=? addr=? terminal=?'
loka 29 19:46:24 koira audit[14113]: AVC apparmor="DENIED" operation="open" profile="snap.chromium.chromium" name="/var/lib/snapd/desktop/icons/" pid=14113 comm="chrome" requested_mask="r" denied_mask="r" fsuid=1000 ouid=0
loka 29 19:46:24 koira kernel: audit: type=1400 audit(1572371184.832:59): apparmor="DENIED" operation="open" profile="snap.chromium.chromium" name="/var/lib/snapd/desktop/icons/" pid=14113 comm="chrome" requested_mask="r" denied_mask="r" fsuid=1000 ouid=0



I also tried touching the file first, and the error log begins with different errors...
loka 29 20:02:33 koira dbus-daemon[4642]: [session uid=1000 pid=4642] Activating via systemd: service name='org.freedesktop.portal.Documents' unit='xdg-document-portal.service' requested by ':1.216' (uid=1000 pid=16167 comm="/snap/bin/chromium " label="unconfined")
loka 29 20:02:33 koira systemd[4622]: Starting flatpak document portal service...
loka 29 20:02:33 koira xdg-document-portal[16225]: error: Failed to load db: invalid gvdb header
loka 29 20:02:33 koira systemd[4622]: xdg-document-portal.service: Main process exited, code=exited, status=2/INVALIDARGUMENT
loka 29 20:02:33 koira systemd[4622]: xdg-document-portal.service: Failed with result 'exit-code'.
loka 29 20:02:33 koira systemd[4622]: Failed to start flatpak document portal service.