There are various reasons why exports fail, but the most common two are: 1) running out of memory and 2) having a filter that somehow got corrupted. For number 2) see here:
For number 1) try rebooting and running the export before starting anything else. Keep an eye on the amount of memory being used. You don’t say what system you are running on, what your machine specifications are or what version of Shotcut you are running, so it is difficult to give any explicit advice.
If you are running out of memory you could export the video in chunks and combine these for a final export.