iPhone backup error - BackupAgent2 - Error uploading "" to "":
From what you’re describing it looks like you have a corrupted backup on your laptop, I assume deleting it will get you to backup your device. What you can do to check is follow these steps:


In the Finder, navigate to your iTunes backups folder 
The backup folder of your device MUST contain at least the following:


Manifest.db or Manifest.mbdb 
Status.plist 
Manifest.plist 
Info.plist



If you have any of these missing, the backup is corrupted or incomplete. In that case, simply delete this backup and start clean with a new iTunes backup.

I’m also thinking it might be worth checking if any iOS app installed on your iPhone contains files with special characters in the filename -  they can cause issues while backing up (APFS related, yes…). 

I’m sure you’re fine with locating the iTunes backup folder, but just in case anyone else is having trouble here’s how to get there:


Open the Finder
In the Main Menu > Go > Hold alt key, Library menu item appears.
Click Library. 
Navigate to Application Support/MobileSync/Backup and locate the iPhone backup. It should be a folder named with your device UDID.