There was an unexpected response from the server. The file may have been uploaded correctly. Please reload the media library or page to check.
First of all, could you try the workaround described in the article below?
Error when uploading theme or image in WordPress
If uploading from the media library fails with the above error message even though the file is under the maximum upload size, the error is due to exceeding the maximum HTTP request body size set on the server. It may have occurred.
The mixhost server is a shared server where multiple customers use the same server, and uploading a huge file from the browser may affect other customers, so this limit is set. I have received it.
I'm sorry to trouble you, but could you please try uploading by another method such as using FTP?