file upload HTTP error on subdomain not addon domain

At my shared hosting, I have a subdomain and an addon domain. When I try to upload a 3MB .mp4 video file to the Media Library on the sub-domain, the upload progress bar goes quickly to 100% and then I get an http error. When I do the same thing at the addon domain, the file uploads fine.
The file upload max size limit is way past 84MB. The WordPress is a fresh install with no plugins. The theme is Twenty Seventeen. Any ideas why this work for the addon domain and not the sub-domain?