WordPress: Unexpected response from the server. The file may have been uploaded successfully. Check in the Media Library or reload the page

The mentioned error is generic, it may refer to more than one possibility, so check this out:

• Maybe you enabled Cloudflare with basic plan in your hosting, if so, Cloudflare limits the upload to 100 MB.
• Maybe you mixed content on your website, with some pages refering to http and other pages refering to https. If so, please check the link: https://websitesetup.org/http-to-https-wordpress.
• Maybe you installed security plugin.

I think my answer is too late. but I would like to post a solution for anyone still looking for a solution for this issue on WordPress running on Nginx. You should add the following directive to set the maximum allowed size in HTTP(server/location) section.

client_max_body_size 10M;

I faced the same error and finally it was fixed. you may installed maximum upload plugins and its configuration is different than your host configuration, for example: if you set max upload from the plugin to 512M and your host configuration is 64, your browser will show your maximum upload as 512M not 64, but later after uploading the unexpected error will display. the solution : remove the max upload plugin as it my conflict with configuration. and if you host your website with shared server service, call the support to update the max upload size for you