I did a new full install of Ubuntu 23.04 (full = all supplemental software options in the installer such as apps and codecs) in a virtual machine, and Shotcut ran just fine. Next, I ran sudo apt install qtcreator to have Ubuntu install its Qt 6 packages, and Shotcut (portable) still works for me (and the incompatible error does not appear). So, something installed on your systems somehow messed things up for Qt 6, but I do not know what or how (KDE is not yet using Qt6). If you know how to use strace you might find something from the system Qt 6.4.2 loaded.