If you encounter the described problem, try deleting the file ~/.config/QtProject.conf. You will lose some dialog settings, but the problem can be solved this way. Do not forget to make a backup copy of this file beforehand in case of failure.
An indirect sign that this file is to blame is its inadequate size (several megabytes).
Very interesting. That might explain why I never experienced the problem but other people did. I do all of my Shotcut work out of discardable LiveUSB Linux Mint sessions, none of which last long enough to build up a large QtProject.conf file. Mine is under 500 bytes. For somebody that has a permanent OS installation, their situation might be different.
For the Linux AppImage version of Shotcut, the file dialogs sometimes take a long time to appear when doing File > Save As or Open. Some people report a lag of up to 30 seconds. Other users were not affected at all. We didn’t have an explanation for the lag up until possibly now.