Can't export after converting a 1:1 square video into a 16x9 version

[Anna Belle2737338225nx]

I am working on a 15-second ad and I initially made it into a 1080x1080 format, and then converted it to a 1920x1080. It exported fine as a square, but when I try to export in the 16x9 format it goes about 1/3 of the way through and then stops. I'm not able to export the second 2/3 separately, either. I've never had this problem before. Have tried updating, restarting, copying into new project, etc. Please help!