Thank you!
This seems to be indeed a file size related issue, the server is not allowed to accept requests. This needs to be resolved at your host unfortunately, we cannot do anything about it.
To verify I tried to test by uploading PDF and other media files above size 2MB, but they all failed, for example: PDF upload
Also tried installing a different plugin (Userpro), to make sure it is not ours causing anyhting, same error: https://i.imgur.com/6jjqVzd.png
Here is an article detailing this issue, and possible resolutions. The easiest would be to increase the post_max_size and upload_max_filesize variables in the php.ini by your host – if the issue is caused by that limitation and not something else (like a firewall or different server config).