Sometimes there is a filter or something in a video clip that shotcut just doesn’t like. So in addition to Brian’s advice, if the export did truly fail and you cannot play the file, try adding 2 markers to split you project in half. Then export marker 1 (the first half). If it fails then split the first half into halves again, and if it fails again check the log, which can sometimes reveal more info. If it succeeds, move on to exporting marker 2. And so on and so forth. In this manner you can (sometimes) eventually identify the offending item. But because you said 7 hours for the export, maybe it would be better to split it into 4 sections, or more. Then each marker export won’t take as long to complete AND you can stitch each marker export together at the end, so the time isn’t completely wasted!
One thing to note, in the thread he linked it is important that you follow the uninstall/reinstall instructions to the letter, including the deletion of the appropriate user folders (since you are on windows).
Good luck.