Quicktime failing to join (or save joined) video clips

I have a pair of video clips (mp4) that I would like to join into 1. The internet tells me that I can do this with Quicktime. Unfortunately, whenever I try exporting joined clips, with clips of any size, I get the same error: "The operation could not be completed; an unknown error occured" If I try to save, rather than export, I cannot. But if I try and close, I'm given an option to save. If I choose to save then, I get this error: Can anyone tell me where I'm going wrong? For full information: there is a two minute video of me running into the problem [here](https://youtu.be/OYKW0RErcxM)