This problem was also occurring with all my themes and plugins installed. I’m wondering if it’s a host issue… although I don’t think it is, because I had the domain merchfans.theweekndbible.com hosted with the same host (no cpanel changes made) and the upload button was working.
Here is the vendor account for testing purposes too.
Vendor Test:3eXYGx@J6BFOmcL@BVb(Wly)