This has been resolved. It was probably a (lack of sufficient) memory problem.
The OP sent me his project - a very large number of files totaling over 10GB of space. I managed to get Shotcut to export it on my Windows 10 desktop that has an SSD, but only 8GB of memory. At several points it got very close to hitting the memory limit.
I started Shotcut and opened the MLT file and then ran the export. I did this immediately after a reboot and made sure that no other applications were running. It got very close (85%) to using the maximum amount of physical memory. At one time I had to minimise Shotcut itself to give the “qmellt.exe” process a bit more memory to play with.