I'm not able to read the issue fully from the first post, but you have issues with changes you have done to dolphin (faulty encryption setup?), see part of the errors from the journalctl log:

sep 23 07:49:40 <HOSTNAME> dolphin[2563]: Couldn't start kuiserver from org.kde.kuiserver.service: QDBusError("org.freedesktop.DBus.Error.ServiceUnknown", "The name org.kde.kuiserver was not provided by any .service files")

    demm
    I have made no changes to dolphin.
    Resume of first post: Updated yesterday without errors. I have no taskbar since I booted this morning. Only one common desktop with laptop and additional display, no icons nor taskbar. Added task launcher plasmoid to work.
    Desktop on secondary monitor does not answer to mouse. Desktop on laptop answers to mouse. Currently the secondary monitor as main display and laptop's as secondary. I swapped them but no changes.
    Still searching for similar cases in internet.

      As the journal shows, there are many errors, leading to the kuiserver failing to load, thus no proper Plasma is loaded.
      Dolphin shows many errors related to inotify_add, example:

      sep 23 06:54:59 <HOSTNAME> dolphin[721]: inotify_add_watch(/run/sudo) failed: (Permission denied)

      leading to the kuiserver failure.
      If you don't use any encryption, then make sure to test with a new, clean user, could very well that one of your settings/themes/plasmoid is not ready for new Qt.

      I tried adding a new user. At first, everything went ok (2 desktops, taskbar...), but when booting today and entering the new user it had the same problem as the old one (no taskbar, one desktop...).
      I will wait a time while saving configurations, mails and data, and then I will reinstall.
      Thank you.

      If it is an issue as bvbfan posted, can you switch to using one monitor for now? I have been subscribed to these bug reports a while now, expect a fix soon. KaOS will use the needed patch right away when they publish it. Reinstalling is no use, if you are dealing with the above bug.

          demm
          Thank for your help.
          I will wait a fix to the bug. I am managing to work in the current situation.
          In fact, now I'm using both monitors with limitations but I can live with it :-)
          Hope for a quick fix

            Any improvement when running kquitapp5 plasmashell && plasmashell once the system has fully booted up?

            Running "kquitapp5 plasmashell && plasmashell" works perfect.
            It recovers my initial configuration of destop. Minor adjustments are needed (some icons lost).
            Until official fix to bug, this solution works great.
            Thank you very much Demm

            • demm replied to this.

              jotade70 @raul338
              Can both of you test patched qt5-base? Patch used is from this bug report:
              https://bugreports.qt.io/browse/QTBUG-86604

              sudo pacman -U http://kaosx.tk/repo/build/qt5-base-5.15.1-2-x86_64.pkg.tar.zst

              Since the patch is not approved yet, this version won't move from the build repo, until it is tested as fixing the multi screen issue.

                  demm
                  I have installed. After reboot I reconfigured the monitor layout and wallpapers again. The config was kept after 3 reboots.

                  I think this fix is working

                    Hello Demm.
                    The patch does not work for me. The system behave just like at the beginning.
                    I keep using the command you listed-

                    One more rebuild for qt5-base is up, this time with the change that started this issue reverted:

                    sudo pacman -U http://kaosx.tk/repo/build/qt5-base-5.15.1-3-x86_64.pkg.tar.zst

                      demm
                      Hello Demm.
                      With the last patch I have recovered the original functionality.
                      Everything seems to work OK after 2 reboots.
                      Thank you.

                        demm Today after a reboot, It happened again!

                        The bug report seems to be fixed with a new QT version. I'll have to try with that compiled

                          A patch is now accepted upstream (no longer taken from any bug report), one more rebuild is up, again, please test & report back:

                          sudo pacman -U http://kaosx.tk/repo/build/qt5-base-5.15.1-4-x86_64.pkg.tar.zst

                            demm
                            No problems with that last patch. Everything works as usual.
                            Regards