Uploading a file stuck at 100% (server error 500)

Please read this guide: How to Increase Maximum Upload File Size in WordPress

The easiest way to figure out the problem is to request error logs from your hosting provider for your website, then examine them around the time when you tried to upload the file.
You will see what limit you have encountered and increasing it will solve the issue.

The settings in PHP that you likely need to adjust are:

• upload_max_filesize – set this to a value > than your backup
• post_max_size – set this to a value > than your backup
• memory_limit – set this to a value > than your backup
• max_execution_time – set this to 0 (infinite)