Problem uploading .vtt files to a Rise course

I uploaded 4 .vtt files to a course this morning. They uploaded very quickly and worked fine. But now, I cannot upload the final 4. The upload process just spins and spins. I've tried other files that did work this morning, but now these just spin as well. I've logged out, logged in, rebooted my machine. re-generated my .vtt files, etc. Any suggestion? Thanks.